U.S. patent application number 16/305540 was filed with the patent office on 2020-07-02 for hollow fiber membrane module.
This patent application is currently assigned to TORAY INDUSTRIES, INC.. The applicant listed for this patent is TORAY INDUSTRIES, INC.. Invention is credited to Masayuki HANAKAWA, Kenta IWAI, Masahiro KIMURA, Tamotsu KITADE, Atsushi KOBAYASHI.
Application Number | 20200206689 16/305540 |
Document ID | / |
Family ID | 60478619 |
Filed Date | 2020-07-02 |
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United States Patent
Application |
20200206689 |
Kind Code |
A1 |
KOBAYASHI; Atsushi ; et
al. |
July 2, 2020 |
HOLLOW FIBER MEMBRANE MODULE
Abstract
This hollow fiber membrane module is provided with: a
cylindrical case having a first end and a second end in the
direction of height; a plurality of hollow fiber membranes
accommodated in the cylindrical case; and a first potting unit
attaching the end parts of the plurality of hollow fiber membranes
positioned at the first end of the cylindrical case while the end
parts are open. The hollow fiber membranes have a rupture strength
of 23 MPa or more. The filling rate for the hollow fiber membranes
is 40-80%.
Inventors: |
KOBAYASHI; Atsushi; (Shiga,
JP) ; IWAI; Kenta; (Shiga, JP) ; HANAKAWA;
Masayuki; (Shiga, JP) ; KITADE; Tamotsu;
(Shiga, JP) ; KIMURA; Masahiro; (Shiga,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TORAY INDUSTRIES, INC. |
Tokyo |
|
JP |
|
|
Assignee: |
TORAY INDUSTRIES, INC.
Tokyo
JP
|
Family ID: |
60478619 |
Appl. No.: |
16/305540 |
Filed: |
May 30, 2017 |
PCT Filed: |
May 30, 2017 |
PCT NO: |
PCT/JP2017/020156 |
371 Date: |
November 29, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 71/34 20130101;
B01D 69/08 20130101; D01F 6/12 20130101; B01D 69/02 20130101; B01D
2325/24 20130101; Y02A 20/131 20180101; B01D 63/02 20130101 |
International
Class: |
B01D 63/02 20060101
B01D063/02; B01D 69/02 20060101 B01D069/02; B01D 69/08 20060101
B01D069/08; B01D 71/34 20060101 B01D071/34 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2016 |
JP |
2016-108318 |
Jun 24, 2016 |
JP |
2016-125527 |
Claims
1. A hollow-fiber membrane module comprising: a cylindrical case
having a first end and a second end in a height direction, a
plurality of hollow-fiber membranes housed in the cylindrical case,
and a first potting part which bonds end parts of the plurality of
hollow-fiber membranes located on the first end side of the
cylindrical case while letting the end parts be open, wherein the
hollow-fiber membrane has a breaking strength of 23 MPa or more,
and a filling ratio of the hollow-fiber membrane is from 40 to
80%.
2. The hollow-fiber membrane module according to claim 1, wherein
the hollow-fiber membrane module is an external pressure-type
hollow-fiber membrane module.
3. The hollow-fiber membrane module according to claim 1, wherein
the hollow-fiber membrane is a hollow-fiber membrane containing a
fluororesin-based polymer, the hollow-fiber membrane has a columnar
texture oriented in a longitudinal direction thereof, a molecular
chain in the columnar texture is oriented in the longitudinal
direction of the hollow-fiber membrane, and a Raman orientation
parameter .nu. of the molecular chain is from 1.5 to 4.0: Raman
orientation parameter=(I1270/I840) parallel/(I1270/I840)
perpendicular (1) wherein: parallel condition: the longitudinal
direction of the hollow-fiber membrane is parallel to a
polarization direction; perpendicular condition: the longitudinal
direction of the hollow-fiber membrane is orthogonal to the
polarization direction; I1270 parallel: the intensity of Raman band
at 1,270 cm.sup.-1 under the parallel condition; I1270
perpendicular: the intensity of Raman band at 1,270 cm.sup.-1 under
the perpendicular condition; I840 parallel: the intensity of Raman
band at 840 cm.sup.-1 under the parallel condition; and I840
perpendicular: the intensity of Raman band at 840 cm.sup.-1 under
the perpendicular condition.
4. The hollow-fiber membrane module according to claim 3, wherein
the columnar texture has a short-side length of from 0.5 to 3 .mu.m
and an aspect ratio of the columnar texture is 3 or more.
5. The hollow-fiber membrane module according to claim 3, wherein a
thickness uniformity of the columnar texture is 0.50 or more.
6. A hollow-fiber membrane module comprising: a cylindrical case
having a first end and a second end in a height direction, a
plurality of hollow-fiber membranes housed in the cylindrical case,
and a first potting part which bonds end parts of the plurality of
hollow-fiber membranes located on the first end side of the
cylindrical case while the end parts being open, wherein the
hollow-fiber membrane has a breaking strength of 25 MPa or more,
and a filling ratio of the hollow-fiber membrane is from 41 to
80%.
7. The hollow-fiber membrane module according to claim 6, wherein
the hollow-fiber membrane module is an external pressure-type
hollow-fiber membrane module.
8. The hollow-fiber membrane module according to claim 6, wherein
the hollow-fiber membrane module is a hollow-fiber membrane
containing a fluororesin-based polymer, the hollow-fiber membrane
has a columnar texture oriented in a longitudinal direction
thereof, at least a part of molecular chains of the
fluororesin-based polymer are oriented in the longitudinal
direction of the hollow-fiber membrane, and in the hollow-fiber
membrane, an orientation degree .pi. calculated based on the
following formula (2) is 0.4 or more and less than 1.0: Orientation
degree .pi.=(180.degree.-H)/180.degree. (2) wherein H is a
half-width (.degree.) of a diffraction intensity distribution in a
circumferential direction of a wide-angle X-ray diffraction
image.
9. The hollow-fiber membrane module according to claim 8, wherein
the columnar texture has a short-side length of from 0.5 to 3 .mu.m
and an aspect ratio of the columnar texture is 3 or more.
10. The hollow-fiber membrane module according to claim 8, wherein
a thickness uniformity of the columnar texture is 0.60 or more.
11. The hollow-fiber membrane module according to claim 8, wherein
the half-width H is a half-width of an intensity distribution
obtained by circumferentially scanning a crystal peak
(2.theta.=20.4.degree.) derived from a (110) plane of
polyvinylidene fluoride in the wide-angle X-ray diffraction
measurement.
12. The hollow-fiber membrane module according to claim 8, wherein
when wide-angle X-ray diffraction measurement is performed at
measurement points at 1 cm intervals in the longitudinal direction
of the hollow-fiber membrane, the orientation degree .pi. is 0.4 or
more and less than 1.0 at 80% or more of the measurement points.
Description
TECHNICAL FIELD
[0001] The present invention relates to a hollow-fiber membrane
module suitable for water purification treatment, industrial water
treatment, wastewater treatment, seawater desalination, and
treatments of various liquids such as fermentation liquid, food and
beverage.
BACKGROUND ART
[0002] In recent years, a separation membrane such as
microfiltration membrane or ultrafiltration membrane is utilized in
various processes including water purification treatment, water
production and wastewater treatment fields, food industry and
medical fields, etc., because it has advantages of energy saving
and space saving and has features of power saving, product quality
enhancement, etc.
[0003] On the other hand, when membrane separation is applied to
raw liquid, a membrane-impermeable substance such as suspended
substances (hereinafter sometimes referred to as "suspended
solids") and organic matters contained in the raw liquid gradually
sticks and deposits on the membrane surface or in a membrane pore
to cause clogging of the separation membrane. As the liquid flow
resistance of the separation membrane is consequently increased,
the power necessary for membrane separation increases, and membrane
separation cannot be eventually performed. When it becomes
impossible to continue the membrane separation, cleaning of the
separation membrane with a chemical solution is generally conducted
so as to recover membrane separation performance, but if clogging
of the separation membrane proceeds rapidly, the frequency of
chemical cleaning is increased, and the processing cost rises.
[0004] Various membrane separation operation techniques have
therefore been developed with an attempt to continuously maintain
the membrane separation performance over a long period of time
while eliminating clogging of the separation membrane. Examples
thereof include back-washing of passing permeated liquid, water,
etc. from the permeation side to the raw liquid side to push out a
substance stuck in a membrane pore or on a membrane surface; air
scrubbing by feeding a gas from the lower part of a hollow-fiber
membrane module to shake the hollow-fiber membrane (i.e.,
separation membrane in hollow fiber form) and physically cleaning
it (see, for example, Patent Document 1): and a flushing method of
flowing raw liquid or a chemical solution at a high linear velocity
in parallel to a membrane surface on the raw liquid side of a
hollow-fiber membrane (see, for example, Patent Document 2).
BACKGROUND ART DOCUMENTS
Patent Documents
[0005] [Patent Document 1] JP-A-11-342320
[0006] [Patent Document 2] JP-A-2010-005615
[0007] [Patent Document 3] Japanese Patent No. 4885539
[0008] [Patent Document 4] WO 03/031038
[0009] [Patent Document 5] JP-A-2006-297383
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0010] In order to continue stable filtration by reducing clogging
of the hollow-fiber membrane, cross-flow filtration of performing
filtration while flowing a raw liquid in parallel to a membrane
surface, air scrubbing, etc. are effective. While, when the
membrane surface linear velocity at the time of cross-flow
filtration or the flow velocity of air scrubbing is increased, the
effect of cleaning the hollow-fiber membrane is intensified, a
stress generated in the hollow-fiber membrane may grow to cause
breakage of the hollow-fiber membrane. An object of the present
invention is to provide a hollow-fiber membrane module allowing for
cross-flow filtration at a high membrane surface linear velocity or
air scrubbing at a high flow velocity.
Means to Solve the Problems
[0011] In order to solve the aforementioned problems, the present
invention provides the following techniques [1] to [12].
[1] A hollow-fiber membrane module including:
[0012] a cylindrical case having a first end and a second end in a
height direction,
[0013] a plurality of hollow-fiber membranes housed in the
cylindrical case, and
[0014] a first potting part which bonds end parts of the plurality
of hollow-fiber membranes located on the first end side of the
cylindrical case while letting the end parts be open, in which
[0015] the hollow-fiber membrane has a breaking strength of 23 MPa
or more, and
[0016] a filling ratio of the hollow-fiber membrane is from 40 to
80%.
[2] The hollow-fiber membrane module according to [1], in which the
hollow-fiber membrane module is an external pressure-type
hollow-fiber membrane module. [3] The hollow-fiber membrane module
according to [1] or [2], in which the hollow-fiber membrane is a
hollow-fiber membrane containing a fluororesin-based polymer,
[0017] the hollow-fiber membrane has a columnar texture oriented in
a longitudinal direction thereof,
[0018] a molecular chain in the columnar texture is oriented in the
longitudinal direction of the hollow-fiber membrane, and
[0019] a Raman orientation parameter .nu. of the molecular chain is
from 1.5 to 4.0:
Raman orientation parameter=(I1270/I840) parallel/(I1270/I840)
perpendicular (1)
(in which:
[0020] parallel condition: the longitudinal direction of the
hollow-fiber membrane is parallel to a polarization direction;
[0021] perpendicular condition: the longitudinal direction of the
hollow-fiber membrane is orthogonal to the polarization
direction;
[0022] I1270 parallel: the intensity of Raman band at 1,270
cm.sup.-1 under the parallel condition;
[0023] I1270 perpendicular: the intensity of Raman band at 1,270
cm.sup.-1 under the perpendicular condition;
[0024] I840 parallel: the intensity of Raman band at 840 cm.sup.-1
under the parallel condition; and
[0025] I840 perpendicular: the intensity of Raman band at 840
cm.sup.-1 under the perpendicular condition).
[4] The hollow-fiber membrane module according to any one of [1] to
[3], in which the columnar texture has a short-side length of from
0.5 to 3 .mu.m and an aspect ratio of the columnar texture is 3 or
more. [5] The hollow-fiber membrane module according to any one of
[1] to [4], wherein a thickness uniformity of the columnar texture
is 0.50 or more. [6] A hollow-fiber membrane module including:
[0026] a cylindrical case having a first end and a second end in a
height direction,
[0027] a plurality of hollow-fiber membranes housed in the
cylindrical case, and
[0028] a first potting part which bonds end parts of the plurality
of hollow-fiber membranes located on the first end side of the
cylindrical case while the end parts being open, in which
[0029] the hollow-fiber membrane has a breaking strength of 25 MPa
or more and
[0030] a filling ratio of the hollow-fiber membrane is from 41 to
80%.
[7] The hollow-fiber membrane module according to 161, wherein the
hollow-fiber membrane module is an external pressure-type
hollow-fiber membrane module. [8] The hollow-fiber membrane module
according to [6] or [7], wherein the hollow-fiber membrane module
is a hollow-fiber membrane containing a fluororesin-based
polymer,
[0031] the hollow-fiber membrane has a columnar texture oriented in
a longitudinal direction thereof,
[0032] at least a part of molecular chains of the fluororesin-based
polymer are oriented in the longitudinal direction of the
hollow-fiber membrane, and
[0033] in the hollow-fiber membrane, an orientation degree .pi.
calculated based on the following formula (2) is 0.4 or more and
less than 1.0:
Orientation degree .pi.=(180.degree.-H)/180.degree. (2)
(in which H is a half-width (.degree.) of a diffraction intensity
distribution in a circumferential direction of a wide-angle X-ray
diffraction image). [9] The hollow-fiber membrane module according
to [8], in which the columnar texture has a short-side length of
from 0.5 to 3 .mu.m and an aspect ratio of the columnar texture is
3 or more. [10] The hollow-fiber membrane module according to [8]
or [9], in which a thickness uniformity of the columnar texture is
0.60 or more. [11] The hollow-fiber membrane module according to
any one of [8] to [10], in which the half-width H is a half-width
of an intensity distribution obtained by circumferentially scanning
a crystal peak (2.theta.=20.4.degree.) derived from a (110) plane
of polyvinylidene fluoride in the wide-angle X-ray diffraction
measurement. [12] The hollow-fiber membrane module according to any
one of [8] to [11], in which when wide-angle X-ray diffraction
measurement is performed at measurement points at 1 cm intervals in
the longitudinal direction of the hollow-fiber membrane, the
orientation degree n is 0.4 or more and less than 1.0 at 80% or
more of the measurement points.
Advantages of the Invention
[0034] In the hollow-fiber membrane module, the breaking strength
of the hollow-fiber membrane is 23 MPa or more, enabling cleaning
at a high membrane surface linear velocity, and the filling ratio
of the hollow-fiber membrane is from 40 to 80%, enabling it to
increase the membrane surface linear velocity even when the flow
rate is constant, so that high cleaning effect can be obtained.
BRIEF DESCRIPTION OF THE INVENTION
[0035] FIG. 1 is a schematic vertical cross-sectional diagram of
the hollow-fiber membrane module according to an embodiment of the
present invention.
[0036] FIG. 2 is an A-A line cross-sectional view of the
hollow-fiber membrane module of FIG. 1.
[0037] FIG. 3 is a schematic diagram illustrating the production
method of the hollow-fiber membrane module of FIG. 1.
[0038] FIG. 4 is a diagram illustrating a photograph of a
cross-section in the longitudinal direction of the hollow-fiber
membrane.
[0039] FIG. 5 is a diagram illustrating a photograph of a
cross-section in the longitudinal direction of the hollow-fiber
membrane of Reference Example 8.
[0040] FIG. 6 is a diagram illustrating a photograph of a
cross-section in the longitudinal direction of the hollow-fiber
membrane of Reference Example 11.
[0041] FIG. 7 is a diagram illustrating the intensity distribution
in the azimuth angle direction at 2.theta.=20.4.degree. of each of
the hollow-fiber membranes of Reference Example 8 and Reference
Example 11.
[0042] FIG. 8 is a diagram illustrating the Raman orientation
parameter at each measurement site of the hollow-fiber membrane of
Reference Example 8.
MODE FOR CARRYING OUT THE INVENTION
[0043] The hollow-fiber membrane module according to an embodiment
of the present invention is described in detail below based on the
drawings. However, the present invention is not limited by this
embodiment. In the present description, mass % and wt % have the
same meaning.
First Embodiment
<Hollow-Fiber Membrane Module>
[0044] The configuration of the hollow-fiber membrane module
according to a first embodiment of the present invention is
described by referring to the drawings. FIG. 1 is a schematic
vertical cross-sectional diagram of the hollow-fiber membrane
module according to a first embodiment of the present
invention.
[0045] The hollow-fiber membrane module 100 illustrated in FIG. 1
includes a cylindrical case 3 being open at both ends, a large
number of hollow-fiber membranes 1 housed in the cylindrical case
3, an upper cap 6 attached to the upper part of the cylindrical
case 3, and a lower cap 7 attached to the lower part of the
cylindrical case 3. Furthermore, the hollow-fiber membrane module
100 includes a first potting part 4, a second potting part 5, etc.
Here, the "upper" and "lower" indicate the top and bottom in a
posture when using the module 100 and correspond to the top and
bottom of FIG. 1.
[0046] On a side surface of the cylindrical case 3, a raw liquid
outlet 10 is provided near the upper end of the cylindrical
case.
[0047] The large number of hollow-fiber membranes 1 are bundled to
form a hollow-fiber membrane bundle 2. The filling ratio of the
hollow-fiber membrane bundle 2 in the cylindrical case 3 is
preferably from 40 to 80%. Details of the filling ratio are
described later.
[0048] The first potting part 4 is also referred to as an upper
potting part. The first potting part 4 is formed of an adhesive and
liquid-tightly and airtightly bonds the upper-side end part
(corresponding to the "first end part") of the hollow-fiber
membrane bundle 2 to the cylindrical case 3 while letting an end
face of the hollow-fiber membrane 1 be open. That is, the
hollow-fiber membrane bundles 2 are bundled by the first potting
part 4 and fixed to the inner wall of the cylindrical case 3.
[0049] The hollow-fiber membrane module 100 further includes a flow
regulating cylinder 12. The flow regulating cylinder 12 is a
tubular member disposed inside of the cylindrical case 3. The flow
regulating cylinder 12 is disposed below the first potting part 4.
The top and bottom of the flow regulating cylinder 12 are open, and
an opening, such as a plurality of slits, is provided on a side
surface. The flow regulating cylinder 12 can pass a liquid through
the opening. The flow regulating cylinder 12 is provided on the
periphery of the raw liquid outlet 10 with the purpose of
preventing the treated raw liquid from channeling. For example, in
the case of performing cross-flow filtration with a hollow-fiber
membrane module without a flow regulating cylinder 12, the flow
velocity of the raw liquid within the cylindrical case 3 is
increased on the raw liquid outlet 10 side (left side of FIG. 1)
and reduced on the side of a surface opposing the raw liquid outlet
10 (right side of FIG. 1) and therefore, the hollow-fiber membrane
cleaning performance may be insufficient on the side of a surface
opposing the raw liquid outlet 10 (right side of FIG. 1). When the
flow regulating cylinder 12 is provided, channeling within the
cylindrical case 3 is prevented, and the hollow-fiber membrane
cleaning performance can thereby be enhanced.
[0050] The second potting part 5 is also referred to as a lower
potting part. The second potting part 5 is formed of an adhesive
and in the lower-side end part (corresponding to the "second end
part") of the hollow-fiber membrane bundle 2, is bonded to the
cylindrical case 3 while sealing the lower end face of the
hollow-fiber membrane 1. More specifically, the second potting part
5 is disposed to face the first potting part 4 within the
cylindrical case 3. Thus, in the lower part of the separation
membrane module, the hollow part of the hollow-fiber membrane
bundle 2 is sealed by an adhesive and is in a state incapable of
opening. The hollow-fiber membrane bundles 2 are bundled by the
second potting part 5 and fixed to the inner wall of the
cylindrical case 3.
[0051] The second potting part 5 has a through hole 11 continuing
from a surface opposing the first potting part 4 to the backward
surface. The through hole 11 has a role as a raw liquid passage or
an air passage at the time of air scrubbing. FIG. 2 is an A-A line
cross-sectional view of the hollow-fiber membrane module 100 of
FIG. 1 and illustrates an example of the arrangement of through
holes 11 in the second potting part 5. In order to prevent a raw
liquid channeling during cross-flow filtration or an air channeling
during air scrubbing, the through holes 11 are preferably arranged
evenly in the second potting part.
[0052] The upper cap 6 has a filtered liquid outlet 9. The upper
cap 6 is liquid-tightly and airtightly attached to the upper part
of the cylindrical case 3. The upper cap 6 is attachable/detachable
relative to the upper part of the cylindrical case 3. The lower cap
7 has a raw liquid inflow port 8. The lower cap 7 is liquid-tightly
and airtightly attached to the lower part of the cylindrical case
3. The lower cap 7 is attachable/detachable relative to the lower
part of the cylindrical case 3.
[0053] The raw liquid flows into the hollow-fiber membrane module
100 through the raw liquid inflow port 8 of the lower cap 7, and a
raw liquid having not passed through the hollow-fiber membrane 1 is
discharged from the raw liquid outlet 10 to the outside of the
hollow-fiber membrane module 100. A filtered liquid having passed
through the hollow-fiber membrane 1 is discharged from the filtered
liquid outlet 9 of the upper cap 6 to the outside of the
hollow-fiber membrane module 100. A system of filtering a raw
liquid in this way while flowing it in parallel to the membrane
surface is referred to as cross-flow filtration and has an effect
of preventing suspended substances, etc. in the raw liquid from
depositing on the membrane surface or an effect of preventing
components contained in the raw liquid from causing concentration
polarization on the membrane surface. In addition, a system of, as
in FIG. 1, feeding a raw liquid to the outer side of the
hollow-fiber membrane and performing filtration from the outer side
to the inner side is referred to as an extemal pressure system.
Conversely, a system of performing filtration from the inner side
to the outer side of the hollow-fiber membrane is referred to as an
internal pressure system.
[0054] In the case of performing cross-flow filtration, when the
membrane surface linear velocity of raw liquid is increased, the
shear stress acting on the membrane surface increases, and the
cleaning performance is enhanced. In the cross-flow filtration, a
raw liquid flows in through the raw liquid inflow port 8 of the
hollow-fiber membrane module 100, and the raw liquid is discharged
from the raw liquid outlet 10. In addition, the filtered liquid is
delivered to the upper part of the hollow-fiber membrane module 100
through the hollow part of the hollow-fiber membrane and discharged
from the filtered liquid outlet 9. The membrane surface linear
velocity of cross-flow filtration is preferably from 0.3 to 5 m/s,
but if the membrane surface linear velocity is increased, the
stress acting on the hollow-fiber membrane increases and therefore,
the hollow-fiber membrane may be broken. Above all, in the case of
an external pressure-type hollow-fiber membrane module 100
illustrated in FIG. 1, the raw liquid flows out from the raw liquid
outlet 10 provided on a side surface of the cylindrical case 3. In
the external pressure-type hollow-fiber membrane module thus having
a raw liquid inflow port or a raw liquid outlet in a direction
perpendicular to the longitudinal direction of the hollow-fiber
membrane, a raw liquid flow is generated in a direction
perpendicular to the longitudinal direction of the hollow-fiber
membrane, and this produces a drag force on the hollow-fiber
membrane. The drag force is proportional to the square of the flow
velocity and therefore, when the membrane surface linear velocity
of cross-flow filtration is increased, a large drag force may be
produced on the hollow-fiber membrane around the raw liquid outlet
10 to cause breakage of the hollow-fiber membrane. In order to
prevent breakage of the hollow-fiber membrane during cross-flow
filtration, the breaking strength of the hollow-fiber membrane is
preferably 23 MPa or more, more preferably 26 MPa or more.
[0055] Incidentally, a smaller diameter of the hollow-fiber
membrane leads to an increase in the specific surface area and is
advantageous in view of membrane area but poses a problem that the
pressure loss at the time of passing of liquid in the hollow part
increases. Accordingly, the inside diameter of the hollow-fiber
membrane is preferably 0.5 mm or more. In addition, in order to
increase the specific surface area of the hollow-fiber membrane,
the outside diameter of the hollow-fiber membrane is preferably 3.0
mm or less. Meanwhile, in the extemal pressure-type hollow-fiber
membrane module, if the transmembrane pressure difference is high,
the hollow-fiber membrane may be buckled. As the outside
diameter/inside diameter ratio of the hollow-fiber membrane is
larger, the pressure resistance is increased and buckling is less
likely to occur. For this reason, the outside diameter/inside
diameter ratio is preferably 1.5 or more.
[0056] In the cross-flow filtration, the membrane surface is
cleaned by a raw liquid stream flowing in parallel to the membrane
surface, but with the same average linear velocity of raw liquid
within the hollow-fiber membrane module, as the distance between
hollow-fiber membranes is smaller, the shear stress acting on the
membrane surface is higher, and the membrane surface cleaning
effect increases. In order to increase the cleaning effect during
cross-flow filtration by reducing the inter-membrane distance
between hollow-fiber membranes, the filling ratio of the
hollow-fiber membrane within the hollow-fiber membrane module is
preferably from 40 to 80%, more preferably from 50 to 70%. When the
filling ratio of the hollow-fiber membrane is 40% or more, the
distance between membranes is reduced, making it possible to
increase the cleaning efficiency at the time of cross-flow
filtration and prevent a rise in the transmembrane pressure
difference. In addition, as the filling ratio of the hollow-fiber
membrane is higher, the membrane surface linear velocity can be
increased with the same flow rate of raw liquid and thus cleaning
effect can be enhanced. Meanwhile, when the filling ratio of the
hollow-fiber membrane is 80% or less, the hollow-fiber membrane is
easily fixed by the potting part.
[0057] The filling ratio of the hollow-fiber membrane as used
herein indicates the proportion of the area occupied by a
hollow-fiber membrane portion in a transverse cross-section (in
FIG. 1, a plane parallel to the horizontal direction and
perpendicular to the paper plane) of the cylindrical case 3 of the
hollow-fiber membrane module between the first potting part and the
second potting part. Denoting S1 as the cross-sectional area of a
hollow-fiber membrane existing portion on the inner side of the
cylindrical case 3 and S2 as the total cross-sectional area of the
hollow-fiber membrane, the filling ratio of the hollow-fiber
membrane can be represented by the following formula (3). Here, in
the case where a member other than the hollow-fiber membrane, such
as flow regulating cylinder 12, is present, the cross-sectional
area obtained by subtracting the cross-sectional area of the member
other than the hollow-fiber membrane from the cross-sectional area
on the inner side of the cylindrical case 3 is denoted by S. In
addition, the nozzle portion on a side surface of the cylindrical
case 3, which is provided as the raw liquid outlet 10, is also not
included in the cross-sectional area S. When an inner-side member
such as flow regulating cylinder 12, a reduced diameter part or an
expanded diameter part is present in the cylindrical case 3, the
cross-sectional area S is changed in that portion. In the present
invention, with respect to the space between the second potting
part-side interface of the first potting part of the hollow-fiber
membrane module and the first potting-side interface of the second
potting part, the cross-sectional area S is calculated for 10 sites
at regular intervals and denoting the average value thereof as the
cross-sectional area S1 of the hollow-fiber membrane existing
portion, the filling ratio of the hollow-fiber membrane is
calculated according to the following formula (3):
Filling ratio [%] of hollow-fiber membrane=S2/S1.times.100 (3)
[0058] Here, the total cross-sectional area S2 of the hollow-fiber
membrane can be represented by the following formula (4). With
respect to 10 hollow-fiber membranes in the hollow-fiber membrane
module, the outside diameter is measured for every two directions
of longest direction and shortest direction, and the average value
of measured values of a total of the 20 sites is designated as the
outside diameter R of the hollow-fiber membrane. Using the outside
diameter R and assuming the hollow-fiber membrane is a perfect
circle, the total cross-sectional area S2 of the hollow-fiber
membrane is calculated according to formula (4):
S2=[circular constant].times.[outside diameter R of hollow-fiber
membrane/2].sup.2.times.[number of hollow-fiber membranes within
hollow-fiber membrane module] (4)
[0059] The above-described average linear velocity of raw liquid
within the hollow-fiber membrane module can be represented by the
following formula (5):
Average linear velocity [m/s]=flow rate of raw liquid
[m.sup.3/s]/(S1-S2)[m.sup.2] (5)
<Potting Method of Hollow-Fiber Membrane Module>
[0060] Bundling hollow-fiber membranes with an adhesive is referred
to as potting. The method for potting includes, as representative
methods, a centrifugal potting method in which a liquid adhesive is
infiltrated among hollow fiber membranes by utilizing centrifugal
force and then cured: and a static potting method in which a liquid
adhesive is fed by a metering pump or head, allowed to naturally
flow and thereby infiltrate among hollow fiber membranes 1, and
then cured. In the centrifugal potting method, an adhesive readily
infiltrates among hollow fiber membranes due to centrifugal force,
and even a high-viscosity adhesive can be used.
<Material of Hollow-Fiber Membrane>
[0061] The material for the hollow-fiber membrane of the
hollow-fiber membrane module of the present invention is not
particularly limited, but a hollow-fiber membrane containing, for
example, a fluororesin-based polymer may be used.
[0062] The fluororesin-based polymer as used in the present
description means a resin containing at least one of a vinylidene
fluoride homopolymer and a vinylidene fluoride copolymer. The
fluororesin-based polymer may contain a plurality of kinds of
vinylidene fluoride copolymers.
[0063] The vinylidene fluoride copolymer is a polymer having a
vinylidene fluoride residue structure and is typically a copolymer
of a vinylidene fluoride monomer and other fluorine-based monomer,
etc. Such a copolymer includes, for example, a copolymer of
vinylidene fluoride and one or more kinds of monomers selected from
vinyl fluoride, tetrafluoroethylene, hexafluoropropylene and
chlorotrifluoroethylene.
[0064] In addition, a monomer other than the above-described
fluorine-based monomer, such as ethylene, may be copolymerized to
the extent not impairing the effects of the present invention.
[0065] The weight average molecular weight of the fluororesin-based
polymer may be appropriately selected according to the strength and
water permeation performance required for the polymer separation
membrane, but as the weight average molecular weight is larger, the
water permeation performance is reduced, and as the weight average
molecular weight is smaller, the strength is reduced. For this
reason, the weight average molecular weight is preferably from
50,000 to 1,000,000. In the case of a water treatment application
where the polymer separation membrane is subject to chemical
cleaning, the weight average molecular weight is preferably from
100,000 to 700,000, more preferably from 150,000 to 600.000.
[0066] The hollow-fiber membrane preferably contains the
fluororesin-based polymer as a main component, and the proportion
of the fluororesin-based polymer in the hollow-fiber membrane is
preferably 80 wt % or more, more preferably 90 wt % or more, still
more preferably 95 wt % or more. The hollow-fiber membrane may be
composed of only the fluororesin-based polymer.
[0067] Here, the "hollow-fiber membrane containing the
fluororesin-based polymer as a main component" can be interchanged
with the "hollow-fiber membrane based on the fluororesin-based
polymer". In the present description, other elements are also
described by the phrase "X contains Y as a main component", and
this can similarly be interchanged with "X is based on Y".
<Columnar Texture>
(a) Dimension
[0068] As illustrated in FIG. 4, the hollow-fiber membrane 1 has a
columnar texture 17 oriented in the longitudinal direction of the
hollow-fiber membrane 1. The "columnar texture" is a solid material
having a uniform thickness and having a shape long in one
direction. The aspect ratio (longitudinal length/short-side length)
of the columnar texture is preferably 3 or more. In FIG. 4, the
columnar structure is photographically shown and therefore, a scale
is indicated, but the present invention is not limited thereto.
[0069] Here, the "longitudinal length" indicates a length in the
longitudinal direction of the columnar texture. The "short-side
length" is an average length in the short-side direction of the
columnar texture. Furthermore, "oriented in the longitudinal
direction" means that out of angles between the longitudinal
direction of the columnar texture and the longitudinal direction of
the hollow-fiber membrane, the acute angle is within
20.degree..
[0070] The longitudinal length and short-side length can be
measured as follows. A hollow-fiber membrane is cut along the
longitudinal direction of the hollow-fiber membrane, and the
obtained cross-section is observed using a scanning electron
microscope (SEM). The magnification is variable according to the
length of the columnar texture and is set to a level allowing a
visual field to include the entire figure of each of 5, preferably
10, columnar textures over its longitudinal direction. In the case
where the length in the longitudinal direction varies in one
columnar texture, a maximum length in the longitudinal direction
may be measured as the longitudinal length. The short-side length
is determined by measuring the length in each short-side direction
at a predetermined number of arbitrary measurement points in one
columnar texture and calculating an average value thereof. The
number of measurement points is a value obtained by dividing the
longitudinal length (.mu.m) by 1 .mu.m (rounded down to the nearest
integer). For example, when the longitudinal length of the columnar
texture is 20.5 .mu.m, the number of measurement points is 20. In
this connection, when the value becomes 21 or more, the length may
be measured at arbitrary 20 points.
[0071] The longitudinal length of the columnar texture is not
particularly limited but is preferably 7 .mu.m or more, more
preferably 10 .mu.m or more, still more preferably 15 .mu.m or
more. The longitudinal length of the columnar texture is, for
example, preferably 50 .mu.m or less, more preferably 40 .mu.m or
less.
[0072] In the present invention, the short-side length of the
columnar texture is preferably from 0.5 to 3 .mu.m. The short-side
length is preferably in the range above, because high strength
performance and high pure-water permeation performance are
obtained. When the short-side length of the columnar texture is 0.5
mun or more, physical strength of the columnar texture itself is
increased and therefore, high strength is obtained. When the
short-side length of the columnar texture is 3 .mu.m or less, the
void among columnar textures becomes large and in turn, good
pure-water permeation performance is obtained. The short-side
length of the columnar texture is more preferably from 0.7 to 2.5
.mu.m, still more preferably from 1 to 2 .mu.m.
[0073] In the hollow-fiber membrane of the present invention,
preferable ranges of representative values of the longitudinal
length and short-side length of the columnar texture are
respectively the same as the above-described preferable ranges of
the longitudinal length and short-side length of each individual
columnar texture. In addition, as for the effects due to each
representative value being in that range, description of effects
when individual columnar textures have a dimension in that range is
applied.
[0074] The representative value of the longitudinal length is
measured as follows. Similar to the measurement of the longitudinal
length, the longitudinal length is measured at 3 sites, preferably
5 sites, in the hollow-fiber membrane for 5, preferably 10,
columnar textures per site. With respect to the obtained values of
the longitudinal length, an average value is determined and can be
used as the representative value of the longitudinal length of the
columnar texture.
[0075] The representative value of the short-side length is
determined by measuring the short-side length (calculated as an
average value) as described above for columnar textures which were
subject to measurement of the representative value of the
longitudinal length, and calculating an average value thereof.
[0076] In the hollow-fiber membrane of the present invention, the
representative value of the aspect ratio of the columnar texture
calculated from the representative value of the longitudinal length
and the representative value of the short-side length is preferably
3 or more, more preferably 5 or more, still more preferably 10 or
more, yet still more preferably 20 or more.
[0077] In the present invention, it is preferred that the
short-side length of the columnar texture is from 0.5 to 3 .mu.m
and the aspect ratio of the columnar texture is 3 or more.
Incidentally, the upper limit of the aspect ratio is not
particularly limited but may be, for example, 50 in consideration
of the existing production method, etc. of the hollow-fiber
membrane.
(b) Thickness Uniformity
[0078] As described later, the hollow-fiber membrane of the present
invention can be produced by forming a hollow fiber from a
membrane-forming solution containing a polymer, and stretching the
hollow fiber. For the sake of convenience, the state before
stretching is referred to as "hollow fiber", and the state after
stretching is referred to as "hollow-fiber membrane".
[0079] The thickness uniformity (the later-described average value
D) of the columnar texture in the hollow-fiber membrane after
stretching is preferably 0.50 or more, more preferably 0.60 or
more, still more preferably 0.70 or more, yet still more preferably
0.80 or more. Although the thickness uniformity is 1.0 at a
maximum, the columnar texture may have a thickness uniformity of
less than 1.0.
[0080] In the hollow-fiber membrane, the columnar texture has a
high thickness uniformity in this way, i.e., a narrowed portion is
little formed in the columnar texture, and the elongation of the
hollow-fiber membrane is thereby increased.
[0081] When the hollow-fiber membrane after stretching keeps high
elongation, this is advantageous in that fiber breakage is less
likely to occur even at the time of an abrupt application of load.
The elongation at break of the hollow-fiber membrane is preferably
50% or more, more preferably 80% or more. The upper limit of the
elongation at break of the hollow-fiber membrane is not
particularly limited but is, for example, 500% in consideration of
the above thickness uniformity.
[0082] The thickness uniformity is described below. As the length
variation among respective short-side directions of the columnar
texture is smaller, a narrowed portion is less formed in the
columnar texture, resulting in high thickness uniformity, and the
columnar texture comes close to a perfect column.
[0083] The thickness uniformity of the columnar texture is
determined by comparing a first cross-section and a second
cross-section each running in parallel to the short-side direction
of the hollow-fiber membrane. This is specifically described
below.
[0084] At the beginning, a first cross-section and a second
cross-section running in parallel to each other are selected. The
distance between the first cross-section and the second cross
section is set to be 5 .mu.m. In each cross-section, a portion
composed of resin and a void portion are distinguished, and the
area of resin portion and the area of void portion are measured.
Next, the area of a portion where when the first cross-section is
projected onto the second cross-section, the portion composed of
resin in the first cross-section and the portion composed of resin
in the second cross-section are overlapped, namely, the overlap
area, is determined. With respect to arbitrary 20 pairs of first
cross-section and second cross-section in one hollow-fiber
membrane, thickness uniformities A and B are determined based on
the following formulae (6) and (7), respectively:
Thickness uniformity A=(overlap area)/(area of resin portion of
second cross-section) (6)
Thickness uniformity B=(overlap area)/(area of resin portion of
first cross-section) (7)
[0085] That is, 20 pairs of thickness uniformities A and B are
obtained for one hollow-fiber membrane. A larger value means that
the thickness of the columnar texture is more uniform. Then, with
respect to each pair, an average value C of thickness uniformities
A and B is calculated. That is, 20 average values C are obtained
for one hollow-fiber membrane. With respect to these average values
C, an average value D is further calculated. The average value D is
the thickness uniformity of this hollow-fiber membrane.
[0086] In the case where 80% or more of 20 average values C
calculated for one hollow-fiber membrane have a value of 0.50 or
more, the hollow-fiber membrane can be said to have a columnar
texture referred to in the present invention.
[0087] In measuring the thickness uniformity, in order to clearly
distinguish the resin portion and the void portion, it is
preferable to previously perform resin-embedding of the
hollow-fiber membrane in an epoxy resin, etc. and staining
treatment of the epoxy resin, etc. with, for example, osmium. By
such resin embedding/staining treatment, the void portion is filled
with an epoxy resin, etc., and at the time of the later-described
cross-sectional processing with a focused ion beam, the portion
composed of a fluororesin-based polymer and the void portion (i.e.,
the epoxy resin portion) can be clearly distinguished, as a result,
high observation accuracy is obtained.
[0088] Furthermore, in order to obtain the above-described first
cross-section and second cross-section each running in parallel to
the short-side direction of the hollow-fiber membrane, a scanning
electron microscope (SEM) equipped with a focused ion beam (FIB) is
preferably used. A face parallel to the short-side direction of the
hollow-fiber membrane is cut out using FIB, and FIB cutting and SEM
observation are repeatedly conducted 200 times at 50 nm intervals
toward the longitudinal direction of the hollow-fiber membrane. By
such continuous cross-sectional observation, information at a depth
of 10 .mu.m can be obtained. Arbitrary first and second
cross-sections forming faces running in parallel to each other and
being spaced 5 .mu.m apart are selected therefrom, and the
thickness uniformities can be determined using formulae (6) and
(7). The observation magnification may be sufficient if it is a
magnification enabling clear identification of a columnar texture
and a spherical texture, and a magnification of, for example, from
1,000 to 5,000 times may be used.
(c) Composition
[0089] The columnar texture preferably contains the
fluororesin-based polymer as a main component, and the proportion
of the fluororesin-based polymer in the columnar texture is
preferably 80 wt % or more, more preferably 90 wt % or more, still
more preferably 95 wt % or more. The columnar texture may be
composed of only the fluororesin-based polymer.
[0090] In other words, the hollow-fiber membrane has a solid matter
containing a fluororesin-based polymer, and at least part of the
solid matter constitutes a columnar texture. All of solid matters
containing a fluororesin-based polymer may constitute a columnar
texture, or part thereof may have a shape not falling under a
columnar texture. In the hollow-fiber membrane, out of solid
matters containing a fluororesin-based polymer, the proportion of
the solid matter constituting a columnar texture is preferably 80
wt % or more, more preferably 90 wt % or more, still more
preferably 95 wt % or more.
(d) Columnar Texture in Hollow-Fiber Membrane
[0091] In the hollow-fiber membrane, the principal structure is
preferably a columnar texture. The proportion of the columnar
texture in the hollow-fiber membrane is preferably 80 wt % or more,
more preferably 90 wt % or more, still more preferably 95 wt % or
more. The hollow-fiber membrane may be composed of only a columnar
texture.
[0092] More specifically, the hollow-fiber membrane preferably has,
as the principal structure, a columnar texture containing a
fluororesin-based polymer as a main component.
[0093] The hollow-fiber membrane can also be phrased as an assembly
of columnar textures.
<Orientation of Molecular Chain>
(a) Raman Orientation
[0094] The orientation of the molecular chain of the columnar
texture constituting the hollow-fiber membrane of the present
invention can be determined by orientation analysis according to
Raman spectroscopy. First, a hollow-fiber membrane is sliced by
cutting with a microtome from a cross-section along the
longitudinal direction of the hollow-fiber membrane. The
thus-obtained section is observed under an optical microscope, and
laser Raman measurement is thereby performed at 1 .mu.m intervals
along the longitudinal direction of a columnar texture while
checking the columnar texture. The number of measurement points in
one columnar texture is a value obtained by dividing the
longitudinal length (.mu.m) of the later-described columnar texture
by 1 .mu.m (rounded down to the nearest integer). For example, when
the longitudinal length of the columnar texture is 20.5 .mu.m, the
number of measurement points is 20.
[0095] Since strong Raman scattering is obtained when the vibration
direction of molecular chain coincides with the polarization
direction of incident light, the orientation degree can be
calculated by appropriately selecting a vibration mode showing a
vibration direction parallel to molecular chain and a vibration
mode showing a vibration direction perpendicular to molecular
chain, and determining the scattering intensity ratio
therebetween.
[0096] For example, in the case where the fluororesin-based polymer
is a polyvinylidene fluoride homopolymer, the Raman band around
1,270 cm.sup.-1 is assigned to a coupling mode of CF.sub.2
(fluorocarbon) stretching vibration and CC (carbon-carbon)
stretching vibration. The vibration direction of these vibrations
is in a mode parallel to molecular chain. Meanwhile, the vibration
direction of the Raman band around 840 cm.sup.-1 is perpendicular
to molecular chain.
[0097] The Raman orientation parameter can therefore be calculated
according to the following formula (1). The Raman orientation
parameter shows a larger value as the orientation in the
longitudinal direction of the hollow-fiber membrane is higher,
shows a value of 1 when non-orientated, and shows a value smaller
than 1 when the orientation in the short-side direction is
high.
Raman orientation parameter=(I1270/I840) parallel/(I1270/I840)
perpendicular (1)
[0098] In formula (1),
[0099] parallel condition: the longitudinal direction of the
hollow-fiber membrane is parallel to the polarization
direction,
[0100] perpendicular condition: the longitudinal direction of the
hollow-fiber membrane is orthogonal to the polarization
direction,
[0101] I1270 parallel: the intensity of Raman band at 1,270
cm.sup.-1 under parallel condition,
[0102] I1270 perpendicular: the intensity of Raman band at 1,270
cm.sup.-1 under perpendicular condition,
[0103] I840 parallel: the intensity of Raman band at 840 cm.sup.-1
under parallel condition, and
[0104] I840 perpendicular: the intensity of Raman band at 840
cm.sup.-1 under perpendicular condition.
[0105] In one hollow-fiber membrane, 10 columnar textures different
from each other, having a length of 0.5 to 1.5 times the
representative value of the longitudinal length of the
later-described columnar texture, are selected. With respect to
each columnar texture, laser Raman measurement is performed at 1
.mu.m intervals as described above, and the Raman orientation
parameters of respective measurement points are calculated
according to formula (1). An average value of the obtained values
is defined as the Raman orientation parameter V.
[0106] In addition, an operation of selecting the largest Raman
orientation parameter and the smallest Raman orientation parameter
among the measurement points of one columnar texture is performed
for 10 columnar textures different from each other. With respect to
selected 10 largest Raman orientation parameters and 10 smallest
Raman orientation parameters, respective average values are
calculated as a maximum Raman orientation parameter M and a minimum
Raman orientation parameter m.
[0107] In order to accurately obtain the Raman orientation
parameter .nu., maximum Raman orientation parameter M, minimum
Raman orientation parameter m and the later-described M/m, the
measurement is preferably performed for 20 columnar textures
different from each other.
[0108] In the hollow-fiber membrane of the present invention, the
Raman orientation parameter .nu. of the molecular chain in the
longitudinal direction of the hollow-fiber membrane is preferably
1.5 or more, 2.0 or more, or 2.5 or more. When the Raman
orientation parameter .nu. is 1.5 or more, the strength of the
hollow-fiber membrane is increased. In addition, the Raman
orientation parameter .nu. is preferably 4.0 or less, or 3.0 or
less.
[0109] It is considered that the maximum Raman orientation
parameter M and the minimum Raman orientation parameter m indicate
respectively the orientation degree at a main orientation site in
the columnar texture and the orientation degree in a portion
working out to a point of effort during stretching.
[0110] Accordingly, M and m may be set to appropriate ranges by
taking into account a balance of performances of the obtained
hollow-fiber membrane, such as strength, elongation and water
permeability. In order to provide high toughness to the
hollow-fiber membrane, M and m are preferably 4.0 or less, more
preferably 3.5 or less, still more preferably 3.0 or less.
Incidentally, the lower limit value is not particularly limited but
is, for example, 1.1.
[0111] It is likely that as the Raman orientation parameter .nu., M
and m are larger, the orientation of molecular chain develops and
the strength of the hollow-fiber membrane increases. On the other
hand, if the ratio M/m of the maximum Raman orientation parameter M
and the minimum Raman orientation parameter m is too large, i.e.,
if the difference in the orientation degree between a portion where
orientation has developed and a portion where orientation has not
developed is too large, a stress concentrates on a portion where
orientation has not developed, and the hollow-fiber membrane is
readily buckled and loses toughness. For this reason, in the
present invention, M/m is preferably from 1.5 to 4.0, more
preferably from 2.0 to 3.5, still more preferably from 2.5 to
3.0.
(b) Orientation Degree in X-Ray Diffraction Measurement
[0112] In the hollow-fiber membrane of the present invention, the
molecular chain of the fluororesin-based polymer is oriented in the
longitudinal direction of the hollow-fiber membrane, and the
orientation degree .pi. of the molecular chain in X-ray diffraction
measurement is less than 0.4, or the molecular chain is
non-oriented. The orientation degree .pi. is calculated from a
half-width H (.degree.) obtained by wide-angle X-ray diffraction
measurement, based on the following formula (2):
Orientation degree .pi.=(180.degree.-H)/180.degree. (2)
(wherein H is a half-width (.degree.) of the diffraction intensity
distribution in the circumferential direction of a wide-angle X-ray
diffraction image).
[0113] The method for measuring the orientation degree .pi. of the
molecular chain in the longitudinal direction of the hollow-fiber
membrane is specifically described below.
[0114] In order to calculate the orientation degree .pi., the
hollow-fiber membrane is fixed to a fiber sample stage by arranging
its longitudinal direction to run vertically. Here, the short-side
direction of the hollow-fiber membrane is a direction parallel to
the diameter direction of the hollow fiber, and the longitudinal
direction is a direction perpendicular to the short-side direction.
The short-side direction can be interchanged with a direction
parallel to the hollow plane, i.e., an in-plane direction of the
hollow plane, and the longitudinal direction can be interchanged
with a direction perpendicular to the hollow plane.
[0115] When X-ray diffraction is performed, an annular diffraction
image called a Debye-Scherrer ring is obtained. In the case of a
non-oriented sample, a great change is not observed in the
diffraction intensity along the Debye-Scherrer ring, but in the
case of an oriented sample, the intensity distribution is deviated
on the Debye-Scherrer ring. Accordingly, the orientation degree can
be calculated from this intensity distribution based on formula
(2).
[0116] More specifically, in the case where the molecular chain is
non-oriented, when 2.theta./.theta. scanning is performed in the
short-side direction (i.e., when a diffraction pattern showing a
diffraction intensity distribution in the diameter direction of
Debye-Scherrer ring is obtained), a peak is observed at a position
around the diffraction angle 2.theta.=20.degree.. The abscissa axis
of the diffraction pattern obtained here is the diffraction angle
2.theta. of X-ray, and the ordinate axis is the diffraction
intensity. Furthermore, when the sample is scanned in the azimuth
angle .beta. direction by fixing the diffraction angle 2.theta. to
the peak position above, i.e., around 20.degree., a diffraction
pattern in which the abscissa axis shows the azimuth angle .theta.
and the ordinate axis shows the diffraction intensity (i.e., a
diffraction intensity distribution along the circumferential
direction of Debye-Scherrer ring at the position of diffraction
angle 2.theta.=20.degree.) is obtained. In the case of a
non-oriented sample, the diffraction intensity is substantially
constant throughout 360.degree. in the circumferential direction of
Debye-Scherrer ring.
[0117] On the other hand, in the case where the molecular chain is
oriented in the longitudinal direction of the hollow-fiber
membrane, a strong diffraction intensity is observed on the azimuth
angle corresponding to the short-side direction of the hollow-fiber
membrane (i.e., on the equatorial line) on the Debye-Scherrer ring
around 2.theta.=20.degree., and a small diffraction intensity is
obtained in other portions. More specifically, in the case of an
oriented sample, the diffraction intensity distribution in the
diameter direction of Debye-Scherrer ring shows a diffraction peak
around 2.theta.=20.degree., similarly to a non-oriented sample, and
the distribution in the circumferential direction shows, unlike a
non-oriented sample, a diffraction peak on the azimuth angle
corresponding to the short-side direction of the hollow-fiber
membrane.
[0118] In the description above, the position of diffraction peak
in the diameter direction of Debye-Scherrer ring (i.e., the value
of 2.theta. corresponding to the diffraction peak) is "around
20.degree.". However, the value of 2.theta. differs depending on
the structure or blending of polymer and may range from 15 to
25.degree.. For example, when X-ray diffraction is performed for a
polyvinylidene fluoride homopolymer having an .alpha. crystal or
.beta. crystal, a diffraction peak derived from a (110) plane of
.alpha. crystal or .beta. crystal, i.e., a plane parallel to
molecular chain, is observed around 2.theta.=20.4.degree..
[0119] As described above, the intensity distribution in the
azimuth angle direction is obtained by fixing the value of
diffraction angle 2.theta. and furthermore, measuring the intensity
in the range from 0.degree. up to 360.degree. in the azimuth angle
direction (circumferential direction). This intensity distribution
may also be said to be an intensity distribution obtained by
scanning a crystal peak on a diffraction image in the
circumferential direction. Here, when the ratio between the
intensity at an azimuth angle of 180.degree. (longitudinal
direction) and the intensity at an azimuth angle of 90.degree.
(short-side direction) is 0.80 or less or 1.25 or more, it is
regarded that a peak is present, and using the intensity
distribution in this azimuth angle direction, the width at a
position of half the peak height (half-width H) is determined.
[0120] In the intensity distribution obtained by scanning a crystal
peak in the circumferential direction, when the ratio between the
intensity at an azimuth angle of 180.degree. and the intensity at
an azimuth angle of 90.degree. is more than 0.80 and less than
1.25, it is regarded that a peak is absent. That is, in this case,
the fluororesin-based polymer is determined to be non-oriented. The
orientation degree .pi. is calculated by substituting the
half-width H into formula (2).
[0121] In the hollow-fiber membrane of the present invention, the
orientation degree .pi. of the molecular chain of the
fluororesin-based polymer in the longitudinal direction of the
hollow-fiber membrane is preferably less than 0.4. Here, the
molecular chain of the fluororesin-based polymer may be
non-oriented relative to the longitudinal direction of the
hollow-fiber membrane. High toughness is obtained when the
hollow-fiber membrane is in the state of small orientation degree,
particularly, in the non-oriented state. Incidentally, when
wide-angle X-ray diffraction measurement is performed at
measurement points at intervals of 1 cm in the longitudinal
direction of the hollow-fiber membrane, it is preferred that at 80%
or more of the measurement points, the orientation degree n of the
molecular chain of the fluororesin-based polymer is less than 0.4
or the molecular chain of the fluororesin-based polymer is
non-oriented.
[0122] In the case where the hollow-fiber membrane contains an
.alpha. crystal or .beta. crystal of polyvinylidene fluoride, the
half-width H is preferably determined from an intensity
distribution obtained by circumferentially scanning a crystal peak
(2.theta.=20.4.degree.) derived from a (110) plane of the .alpha.
crystal or .beta. crystal of polyvinylidene fluoride in wide-angle
X-ray diffraction measurement.
[0123] There is a tendency that the orientation degree .pi.
determined by wide-angle X-ray diffraction measurement represents
the orientation of molecular chain of the entire porous
hollow-fiber membrane and the Raman orientation parameter .nu.
determined by Raman spectroscopy represents the orientation of
molecular chain when focus is directed onto the columnar texture of
the porous hollow-fiber membrane, i.e., the orientation of local
molecular chain. In the hollow-fiber membrane of the present
invention, crystal orientation of the entire porous hollow-fiber
membrane in wide-angle X-ray diffraction is not observed, but the
local molecular chain in Raman spectroscopy is in the oriented
state, so that both high strength and high toughness can be
achieved.
[0124] It is preferred that the orientation degree .pi. by
wide-angle X-ray diffraction is less than 0.4 or the molecular
chain is non-oriented and the Raman orientation parameter .nu. by
Raman spectroscopy is 1.5 or more, and it is more preferred that
the Raman orientation parameter .nu. is 2.0 or more.
<Porosity>
[0125] In the hollow-fiber membrane of the present invention, in
order to satisfy both high pure-water permeation performance and
high strength, the porosity is preferably from 40 to 80%, more
preferably from 45 to 75%, still more preferably from 50 to 70%. If
the porosity is less than 40%, the pure-water permeation
performance is reduced, whereas if it exceeds 80%, the strength
significantly decreases and therefore, the membrane lacks
suitability as a hollow-fiber membrane for water treatment.
[0126] The porosity of the hollow-fiber membrane is determined
according to the following formula (8) by using the area of resin
portion and the area of void portion in the above-described
cross-section. In order to increase the accuracy, it is preferable
to determine the porosity for arbitrary 20 or more, preferably 30
or more, cross-sections and use an average value thereof.
Porosity (%)={100.times.(area of void portion)}/{(area of resin
portion)+(area of void portion)} (8)
<Young's Modulus>
[0127] The hollow-fiber membrane of the present invention
preferably has high toughness suitable for practical use, and the
toughness can be denoted by the Young's modulus of a tensile test.
The Young's modulus of the hollow-fiber membrane may be selected
according to use of the hollow-fiber membrane but is preferably
0.15 GPa or more and less than 0.40 GPa, more preferably 0.22 GPa
or more and less than 0.38 GPa, still more preferably 0.24 GPa or
more and less than 0.36 GPa. If the Young's modulus falls below
0.15 GPa, the hollow-fiber membrane is likely to be deformed due to
a stress applied during use. In addition, if the Young's modulus is
0.40 GPa or more, when the hollow membrane is shaken, for example,
by scrubbing cleaning, etc. which is frequently conducted in the
application for water treatment, the yean breakage in hollow-fiber
membrane readily occurs
<Others>
[0128] The hollow-fiber membrane of the present invention may
contain a texture other than the above-described columnar texture
to the extent not departing from the object of the present
invention. The structure other than the columnar texture includes,
for example, a spherical texture having an aspect ratio
(longitudinal length/short-side length) of less than 3. The
short-side length and longitudinal length of the spherical texture
are preferably from 0.5 to 3 .mu.m. In the case of using a
spherical texture, as long as the short-side length and
longitudinal length thereof are in the range above, reduction in
the strength of the hollow-fiber membrane can be prevented, and
good pure-water permeation performance can be maintained.
[0129] However, if the proportion of such a spherical texture
having an aspect ratio of less than 3 in the hollow-fiber membrane
is increased, there arises a tendency that spherical textures are
increasingly coupled with each other to increase the narrowed
portion and it is difficult to perform high-ratio stretching or
keep the elongation after stretching. For this reason, a smaller
proportion of the spherical texture in the hollow-fiber membrane is
more preferred, and the proportion is preferably less than 20%,
more preferably less than 10%, still more preferably less than 1%,
i.e., almost nil. It is most preferred that the spherical texture
is not present at all.
[0130] Here, the occupancy (%) of each texture is determined
according to the following formula (9) after taking a photograph of
a cross-section in the longitudinal direction of the porous
hollow-fiber membrane by means of SEM, etc. at a magnification
enabling clear identification of a columnar texture and a spherical
texture, preferably at a magnification of 1,000 to 5.000 times. In
order to increase the accuracy, it is preferable to determine the
occupancy for arbitrary 20 or more, preferably 30 or more,
cross-sections and calculate an average value thereof.
Occupancy (%)={(area occupied by each texture)/(area of entire
photograph)}.times.100 (9)
[0131] Incidentally, the area of the entire photograph and the area
occupied by a texture can be determined preferably by employing,
for example, a method of converting the area into a weight
corresponding to each texture photographed. That is, the photograph
taken may be printed on paper, and the weight of paper
corresponding to the entire photograph and the weight of paper
corresponding to a texture portion cut out therefrom may be
measured. In addition, before taking a photograph by SEM, etc., the
above-described resin embedding/staining treatment and FIB cutting
are preferably applied, because the observation accuracy
increases.
[0132] The hollow-fiber membrane of the present invention may be a
membrane in which a layer having the above-described columnar
texture and a layer having other structure are stacked to the
extent not departing from the object of the present invention.
However, if the thickness of the layer having other structure is
large compared with the layer having the columnar texture, the
object and effects of the present invention can hardly be exerted
and therefore, the ratio of the thickness of the layer having other
structure to the thickness of the layer having the columnar texture
is preferably 0.3 or less, more preferably 0.2 or less.
[0133] In the hollow-fiber membrane of the present invention, it is
preferred that the pure-water permeation performance at 50 kPa and
25.degree. C. is 0.7 m.sup.3/m.sup.2/hr or more and the breaking
strength is 23 MPa or more, and it is more preferred that the
pure-water permeation performance at 50 kPa and 25.degree. C. is
0.7 m.sup.3/m.sup.2/hr or more and the breaking strength is 25 MPa
or more. Above all, from the viewpoint of providing a
high-performance hollow-fiber membrane satisfying both high
pure-water permeation performance and high strength performance, it
is preferred that the pure-water permeation performance at 50 kPa
and 25.degree. C. is from 0.7 to 5.0 m.sup.3/m.sup.2/hr and the
breaking strength is from 23 to 70 MPa, and it is more preferred
that the pure-water permeation performance at 50 kPa and 25.degree.
C. is from 0.7 to 5.0 m.sup.3/m.sup.2/hr and the breaking strength
is from 30 to 60 MPa.
[0134] The measurement of pure-water permeation performance is
performed by manufacturing a miniature module of 200 mm in length
including 4 porous hollow-fiber membranes. External pressure
dead-end filtration of reverse osmosis membrane filtrate is
performed for 10 minutes under the conditions of a temperature of
25.degree. C. and a filtration pressure difference of 16 kPa, and
the permeation amount (m.sup.3) is determined. The permeation
amount (m.sup.3) is converted into a value per unit time (h) and
effective membrane area (m.sup.2), further multiplied by (50/16),
and thereby converted into a value at a pressure of 50 kPa to
determine the pure-water permeation performance.
[0135] The methods for measuring the breaking strength and the
elongation at break are not particularly limited but, for example,
using a tensile tester, a tensile test of a sample having a
measurement length of 50 mm is performed 5 or more times by
changing the sample at a tensile speed of 50 mm/min, and the
breaking strength and the elongation at break can be measured by
determining average values thereof.
[0136] The hollow-fiber membrane described above has sufficient
pure-water permeation performance, strength and elongation for
water purification treatment, industrial water treatment,
wastewater treatment, seawater desalination, and treatment of
various liquids such as fermented liquid, food and beverage.
<Production Method of Hollow-Fiber Membrane>
[0137] The method for producing the hollow-fiber membrane of the
present invention is described below by way of example. The method
for producing a hollow-fiber membrane includes at least:
[0138] 1) a step of forming a hollow fiber having a columnar
texture from a membrane-forming solution containing a
fluororesin-based polymer by thermally induced phase separation, in
which the columnar texture is oriented in the longitudinal
direction and has a thickness uniformity of 0.50 or more and less
than 1.00; and
[0139] 2) a step of stretching the porous hollow fiber obtained in
1) above to 1.8 to 2.7 times in the longitudinal direction at a
stretching speed of 1 to 150%/sec.
(a) Preparation of Membrane-Forming Solution
[0140] The production method of the porous hollow-fiber membrane in
the present invention further includes a step of preparing a
fluororesin-based polymer solution. A fluororesin-based polymer
solution (i.e., a membrane-forming solution containing a
fluororesin-based polymer) is prepared by dissolving a
fluororesin-based polymer in a poor or good solvent for the
fluororesin-based polymer at a relatively high temperature of not
less than the crystallization temperature.
[0141] When the polymer concentration in the membrane-forming
solution is high, a hollow-fiber membrane having high strength is
obtained. On the other hand, when the polymer concentration is low,
the porosity of the hollow-fiber membrane is increased, and the
pure-water permeation performance is enhanced. Accordingly, the
concentration of the fluororesin-based polymer is preferably from
20 to 60 wt %, more preferably from 30 to 50 wt %.
[0142] In the present invention, the poor solvent is a solvent in
which the fluororesin-based polymer cannot be dissolved to a
concentration of 5 wt % or more at a low temperature of 60.degree.
C. or less but can be dissolved to a concentration of 5 wt % or
more in a high-temperature region between 60.degree. C. or more and
not more than the melting point of the fluororesin-based polymer
(for example, when the polymer is composed of a vinylidene fluoride
homopolymer alone, about 178.degree. C.). The good solvent is a
solvent in which the fluororesin-based polymer can be dissolved to
a concentration of 5 wt % or more even in a low-temperature region
of 60.degree. C. or less. The nonsolvent is defined as a solvent in
which the fluororesin-based polymer is neither dissolved nor
swollen at a temperature up to the melting point of the
fluororesin-based polymer or the boiling point of the solvent.
[0143] The poor solvent for the fluororesin-based polymer includes
cyclohexanone, isophorone, .gamma.-butyrolactone, methyl isoamyl
ketone, propylene carbonate, dimethylsulfoxide, etc., and a mixed
solvent thereof. The good solvent includes N-methyl-2-pyrrolidone,
dimethylacetamide, dimethylformamide, methyl ethyl ketone, acetone,
tetrahydrofuran, tetramethylurea, trimethyl phosphate, etc., and a
mixed solvent thereof. The nonsolvent includes water, hexane,
pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride,
o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene
glycol, triethylene glycol, propylene glycol, butylene glycol,
pentanediol, hexanediol, an aliphatic hydrocarbon such as
low-molecular-weight polyethylene glycol, an aromatic hydrocarbon,
an aliphatic polyhydric alcohol, an aromatic polyhydric alcohol, a
chlorinated hydrocarbon, other chlorinated organic liquids, a mixed
solvent thereof, etc.
(b) Formation of Hollow Fiber
[0144] In the hollow fiber forming step, a hollow fiber is obtained
from a membrane-forming solution containing a fluororesin-based
polymer by utilizing a thermally induced phase separation method of
inducing phase separation by temperature change. In order to
perform the later-described high-ratio stretching of 1.8 times or
more, it is preferred that the hollow fiber has a columnar texture
oriented in its longitudinal direction and the thickness uniformity
of the columnar texture is 0.50 or more and less than 1.00. The
lower limit of the thickness uniformity of the columnar texture is
more preferably 0.60 or more, still more preferably 0.70 or more,
yet still more preferably 0.80 or more.
[0145] In the thermally induced phase separation method, two kinds
of phase separation mechanisms are mainly utilized. One is a
liquid-liquid phase separation method in which a polymer solution
dissolved uniformly at a high temperature is separated into a
polymer-rich phase and a polymer-poor phase due to reduction in the
dissolving ability of the solution during a temperature drop and
the structure is thereafter fixed by crystallization. Another is a
solid-liquid phase separation method in which a polymer solution
dissolved uniformly at a high temperature is phase-separated into a
polymer solid phase and a solvent phase due to occurrence of
crystallization of the polymer during a temperature drop.
[0146] A three-dimensional network structure is mainly formed in
the former method, and a spherical structure constituted by a
spherical texture is mainly formed in the latter method. In the
production of the hollow-fiber membrane of the present invention,
the latter phase separation mechanism is preferably utilized.
Accordingly, a polymer concentration and a solvent, inducing
solid-liquid phase separation, are selected. In the former phase
separation mechanism, it is difficult to develop the
above-described columnar texture oriented in the longitudinal
direction of the hollow fiber membrane. This is because the
polymer-rich phase forms a very fine phase by phase separation
before the structure is fixed, and cannot be made columnar.
[0147] As a specific method, a hollow part-forming liquid is
discharged through an inner tube of a double tube-type spinneret
for spinning of a hollow-fiber membrane while discharging the
above-described membrane-forming solution through an outer tube of
the double tube-type spinneret. The thus-discharged
membrane-forming solution is cooled and solidified in a cooling
bath to obtain a hollow-fiber membrane.
[0148] The fluororesin-based polymer solution is, before being
discharged from the spinneret, held in a specific temperature
condition for a given time under pressure. The pressure is
preferably 0.5 MPa or more, more preferably 1.0 MPa or more. The
temperature T of the polymer solution preferably satisfies
Tc+35.degree. C..ltoreq.T.ltoreq.Tc+60.degree. C., more preferably
satisfies Tc+40.degree. C..ltoreq.T.ltoreq.Tc+55.degree. C. Tc is
the crystallization temperature of the fluororesin-based polymer
solution. The time for which the polymer solution is held under
these pressure and temperature is preferably 10 seconds or more,
more preferably 20 second or more.
[0149] Specifically, at any site of a solution feed line for
delivering the polymer solution to the spinneret, a retention part
for allowing the polymer solution to stay is provided, and a
pressurizing unit for applying a pressure to the retained polymer
solution and a temperature-adjusting unit for adjusting the
temperature of the retained polymer solution (for example, a
heating unit) are provided. The pressurizing unit is not
particularly limited, but by disposing two or more pumps in the
solution feed line, a pressure can be applied to any site
therebetween. The pump includes a piston pump, a plunger pump, a
diaphragm pump, a wing pump, a gear pump, a rotary pump, a screw
pump, etc., and two or more kinds of pumps may be used.
[0150] Through this step, a pressure is applied under the
conditions in which crystallization easily takes place, and
therefore, it is presumed that crystal growth has anisotropy and in
turn, not an isotropic spherical structure but a texture oriented
in the longitudinal direction of the hollow fiber membrane is
developed, as a result, a columnar structure is obtained.
[0151] Here, the crystallization temperature Tc of the
fluororesin-based polymer solution is defined as follows. In an
apparatus for differential scanning calorimetry (DSC measurement),
a mixture having the same composition as the composition of the
membrane-forming polymer solution containing a fluororesin-based
polymer, a solvent, etc. is sealed in a sealing type DSC container
and uniformly dissolved by raising the temperature to a dissolution
temperature at a temperature rise rate of 10.degree. C./min and
holding the temperature for 30 minutes, and a rise temperature of a
crystallization peak observed in the process of thereafter lowering
the temperature at a temperature drop rate of 10.degree. C./min is
Tc.
[0152] The cooling bath for cooling the fluororesin-based polymer
solution discharged from the spinneret is described below. In the
cooling bath, a mixed liquid including a poor or good solvent in a
concentration of 50 to 95 wt % and a nonsolvent in a concentration
of 5 to 50 wt % is preferably used. As the poor solvent, use of the
same poor solvent as that in the polymer solution is preferably
employed. For the hollow part-forming liquid, as with the cooling
bath, a mixed liquid including a poor or good solvent in a
concentration of 50 to 95 wt % and a nonsolvent in a concentration
of 5 to 50 wt/t % is preferably used. As the poor solvent, use of
the same poor solvent as that in the polymer solution is preferably
employed.
[0153] Here, in order to develop not a fibrous texture having a
large number of narrowed portions but a columnar texture having a
uniform thickness, it is preferable to promote polymer
uptake/growth into the narrowed portion. The present inventors have
found that the polymer uptake/growth into the narrowed portion
leads to disappearance of a narrowed portion having high interface
energy and can energetically stabilize and therefore be caused to
preferentially occur over the growth in portions other than the
narrowed portion, and intensive studies have been made on the
method for enhancing the thickness uniformity.
[0154] As a result, it has been found that as the method for
uptaking polymer into the narrowed portion and thereby promoting
the texture growth, the thermally induced phase separation
preferably includes at least one of the following cooling steps a)
and b):
[0155] a) a step of soaking the membrane-forming solution in a
cooling bath at a temperature Tb satisfying Tc-30.degree.
C.<Tb.ltoreq.Tc; and
[0156] b) a step of soaking the membrane-forming solution in a
cooling bath at a temperature Tb1 satisfying
Tb1.ltoreq.Tc-30.degree. C., followed by soaking in a cooling bath
at a temperature Tb2 satisfying Tc-30.degree.
C.<Tb2.ltoreq.Tc,
(wherein Tc is the crystallization temperature of the
membrane-forming solution containing a fluororesin-based
polymer).
[0157] In the present invention, it has been found that as the
method a), when cooling/solidification in a cooling bath is
performed near the crystallization temperature of the polymer
solution, the cooling/solidification slowly proceeds. In this case,
denoting Tc as the crystallization temperature of the
fluororesin-based polymer solution, the temperature Tb of the
cooling bath is set to satisfy Tc-30.degree. C.<Tb.ltoreq.Tc,
and Tc-20.degree. C.<Tb.ltoreq.Tc is more preferred.
[0158] The passing time in the cooling bath (i.e., soaking time in
the cooling bath) is not particularly limited as long as enough
time to complete the thermally induced phase separation including
polymer uptake/growth into the narrowed portion can be ensured, and
the passing time may be experimentally determined by taking into
account the number of hollow-fiber membranes, the spinning speed,
the bath ratio, the cooling capacity, etc.
[0159] However, in order to achieve thickness uniformity, the
passing time is preferably set to be as long as possible in the
above-described temperature range of the cooling bath and may be,
for example, 10 seconds or more, preferably 20 seconds or more,
more preferably 30 seconds or more.
[0160] In addition, as the method b), two or more stages of cooling
may be performed. Specifically, the cooling step may include a step
of cooling the solution by using a first cooling bath for promoting
generation/growth of a crystal nucleus by increasing the
supercooling degree, and a step of thereafter cooling the solution
by using a second cooling bath for promoting polymer uptake/growth
into the narrowed portion. The cooling step by the second cooling
bath utilizes a phenomenon that the polymer uptake/growth into the
narrowed portion preferentially occurs mainly in the structure
coarsening process of the phase separation.
[0161] In this case, when the temperature Tb1 of the first cooling
bath for cooling the fluororesin polymer solution discharged from
the spinneret satisfies Tb1.ltoreq.Tc-30.degree. C., the generation
and growth of a crystal nucleus can be promoted by increasing the
supercooling degree, and when the temperature Tb2 of the second
cooling bath is set to a temperature near the crystallization
temperature (specifically, set to satisfy Tc-30.degree.
C.<Tb2.ltoreq.Tc, preferably Tc-20.degree. C.<Tb2.ltoreq.Tc),
the polymer uptake/growth into the narrowed portion can be
promoted. Tc is the crystallization temperature of the polymer
solution.
[0162] The passing time in each cooling bath can be varied, but it
is favorable to set, for example, the passing time in the first
cooling bath to be from 1 to 20 seconds, preferably from 3 to 15
seconds, more preferably from 5 to 10 seconds, and set the passing
time in the second cooling bath to be 10 seconds or more,
preferably 20 seconds or more, more preferably 30 seconds or
more.
[0163] When a texture having a thickness uniformity of less than
0.50 is referred to as "fibrous texture" so as to distinguish it
from the columnar texture, the hollow-fiber membrane disclosed in
JP-A-2006-297383 (Patent Document 5) is a hollow-fiber membrane
having a fibrous texture. Such a porous hollow-fiber membrane
having a fibrous texture is relatively excellent in strength and
pure-water permeation performance, and the present inventors have
therefore attempted to increase the strength by stretching this
membrane. However, it has been found that the membrane cannot be
uniformly stretched and the strength cannot be increased.
[0164] In general, a porous membrane used for water treatment has a
large number of void parts for passing water and since destruction
of the texture proceeds starting from a void part at the time of
stretching, the stretching itself is very difficult. This tendency
is prominent in particular when the hollow-fiber membrane has a
phase-separation porous structure obtained by dry-wet spinning
utilizing a principle of nonsolvent induced phase separation or
thermally induced phase separation, because a large number of fine
voids are present and the porosity is high.
[0165] In the case of the porous membrane having a fibrous texture
described in Patent Document 5, it is considered that stress during
stretching is dispersed by the fibrous texture oriented in the
longitudinal direction and stretching can be performed. However, a
great enhancement of the breaking strength is not achieved, and
intensive studies on the cause thereof have revealed that a fibrous
texture has many narrowed portions and because stress is
concentrated at the narrowed portion during stretching and the
narrowed portion is therefore preferentially stretched, the entire
fibrous texture cannot be uniformly stretched, making it impossible
to increase the stretch ratio.
[0166] In contrast, the present inventors have found that when the
hollow fiber is a hollow fiber having a columnar texture with
uniform thickness, the entire columnar texture can be uniformly
stretched. Such uniform and high-ratio stretching has yielded
success in stretching and orienting the molecular chain of the
fluororesin-based polymer in the longitudinal direction of the
porous hollow-fiber membrane and success in increasing the strength
while maintaining high pure-water permeation performance.
<Stretching>
[0167] In the present invention, the hollow-fiber membrane
including a fluororesin-based polymer and having a columnar
texture, obtained by the method above, is stretched at a low speed
and high ratio, and the molecular chain of the polymer is thereby
oriented in the longitudinal direction of the hollow-fiber
membrane. As a result, a Raman orientation parameter .nu. and an
orientation degree in X-ray diffraction, each in the
above-described range, are realized.
[0168] The stretch ratio is preferably from 1.8 to 2.4 times, more
preferably from 1.9 to 2.3 times. When the stretch ratio is 1.8
times or more, the molecular chain can be sufficiently oriented by
stretching and therefore, the strength of the hollow-fiber membrane
can be increased. In addition, when the stretch ratio is 2.4 times
or less, the pore size of the hollow-fiber membrane does not become
too small, so that high water permeability can be realized and the
elongation and toughness can be maintained.
[0169] In the present invention, the stretching speed is preferably
from 1 to 150%/sec. more preferably from 3 to 100%/sec, still more
preferably from 5 to 50%/sec. When the stretching speed is 1%/sec
or more, the membrane can be stretched without extremely increasing
the size of the stretching treatment equipment. In addition, when
the stretch ratio is 150%/sec or less, the membrane can be
homogeneously stretched stably.
[0170] The hollow fiber having a columnar texture is stretched at
the above-described low speed, and homogeneous stretching of the
entire hollow fiber can thereby be achieved, as a result,
homogeneous orientation can proceed. This homogeneous stretching is
considered to include homogeneous stretching of the entirety of one
columnar texture and stretching of a plurality of different
columnar textures to the same extent.
[0171] As described above, the columnar texture is formed by
uptaking the polymer into the narrowed portion of a solid matter
that has been previously formed. Since the growth rate differs
between the previously formed solid matter and the portion formed
thereafter, the microscopic structure (for example, the number of
molecular chain entanglements per volume) is considered to differ.
Accordingly, the hollow fiber is preferably stretched at a low
speed also for preventing breakage and achieving uniform
stretching.
[0172] The stretching speed is calculated as follows.
Stretching speed (%/sec)=(stretch ratio.times.100-100)/stretching
time(sec)
[0173] Here, the stretch ratio is calculated by "length (m) after
stretching+length (m) before stretching". For the stretching time,
the time (sec) substantially used for stretching is employed. The
stretch ratio may also be calculated from the set speed of the
stretching apparatus, but it is better to dye the hollow-fiber
membrane immediately before stretching to a length of 10 cm in its
longitudinal direction, conduct stretching, and measure the length
of the colored portion before and after the stretching. On this
occasion, the time actually used for stretching can also be
measured.
[0174] The stretching temperature is preferably from 60 to
140.degree. C., more preferably from 70 to 120.degree. C., still
more preferably from 80 to 100.degree. C. When stretching is
performed in an atmosphere of 60.degree. C. or more, the hollow
fiber can be stably and homogeneously stretched. In addition, when
the stretching temperature is 140.degree. C. or less, the
fluororesin-based polymer can be prevented from melting and can be
stretched and oriented. Stretching in a liquid is preferred because
of ease of temperature control, but the stretching may also be
performed in a gas such as steam. As the liquid, water is simple
and preferred, but in the case of stretching the hollow fiber at
about 90.degree. C. or more, use of a low-molecular-weight
polyethylene glycol, etc. may also be preferably employed.
<Cross-Flow Filtration Method, Transmembrane Pressure
Difference>
[0175] In the cross-flow filtration, a raw liquid flows in through
the raw liquid inflow port 8 of the hollow-fiber membrane module
100, and the raw liquid is discharged from the raw liquid outlet
10. In addition, the filtered liquid is delivered to the upper part
of the hollow-fiber membrane module 100 through the hollow part of
the hollow-fiber membrane and discharged from the filtered liquid
outlet 9.
[0176] As described above, in the cross-flow filtration, the raw
liquid flows in parallel to the membrane surface. At this time, the
membrane surface linear velocity may be appropriately set in
accordance with the property of the raw liquid but is preferably
from 0.3 to 5 m/s.
[0177] The filtration flux at the time of cross-flow filtration may
be appropriately set in accordance with the property of the raw
liquid but is preferably from 0.1 to 5.0 m.sup.3/m.sup.2/d, more
preferably from 0.3 to 3.0 m.sup.3/m.sup.2/d.
[0178] In the filtration by a separation membrane, the clogging
state of the separation membrane can be determined from the
transmembrane pressure difference obtained by subtracting the
pressure on the filtered liquid side from the pressure on the raw
liquid side of the separation membrane, and in the case of dead-end
filtration, the transmembrane pressure difference can be calculated
using a manometer on the upstream side of the raw liquid inflow
port 8 of the hollow-fiber membrane module 100 and a pressure on
the downstream side of the filtered liquid outlet 9. If the
filtration flux is the same, when clogging of the separation
membrane is developed, the transmembrane pressure difference rises.
However, in the case of cross-flow filtration, the pressure loss at
the time of passing of raw liquid through the raw liquid-side
passage of the hollow-fiber membrane module 100 is large, and the
transmembrane pressure difference calculated by the above-described
method also contains a pressure loss in the raw liquid-side passage
and therefore, is difficult to appropriately calculate.
Accordingly, denoting P1 as the filtered liquid-side pressure when
filtration is stopped while feeding raw liquid to the raw liquid
side of the hollow-fiber membrane module 100 and denoting P2 as the
filtered liquid-side pressure when filtration is preformed while
feeding raw liquid to the raw liquid side of the hollow-fiber
membrane module 100, .DELTA.P that is a value obtained by
subtracting P2 from P1 may be used as the transmembrane pressure
difference at the time of cross-flow filtration.
<Backwashing Method>
[0179] In the cross-flow filtration, backwashing can also be
conducted by periodically stopping filtration. Backwashing is
conducted to clean the membrane by feeding a backwashing solution
from the filtered liquid outlet 9 of the hollow-fiber membrane
module 100 and flowing the backwashing solution toward the outer
side from the inner side of the hollow-fiber membrane. When the
water permeability is recovered by backwashing, the filtration time
can be extended, and the frequency of chemical cleaning decreases,
so that the operation cost can be reduced. The backwashing can be
conducted with the filtered liquid, or other liquid such as water
may also be used.
[0180] The backwashing flux during backwashing may be appropriately
set in accordance with the property of raw liquid or the clogging
state of separation membrane but is preferably from 1.0 to 10.0
m.sup.3/m.sup.2/d, more preferably from 1.5 to 5.0
m.sup.3/m.sup.2/d. If the backwashing flux is less than 1.0
m.sup.3/m.sup.2/d, this is disadvantageous in that the cleaning
effect is decreased. If the backwashing flux exceeds 10.0
m.sup.3/m.sup.2/d, this is disadvantageous in that the power cost
increases and a large amount of solution needs to be used for
backwashing.
<Air Scrubbing Method>
[0181] In air scrubbing, cleaning is effected by a shear stress due
to air bubbles or shaking of the hollow-fiber membrane in the
course of introducing compressed air through the raw liquid inflow
port 8 of the hollow-fiber membrane module 100 and discharging the
air from the raw liquid outlet 10. The air feed flow rate in air
scrubbing varies depending on the area in a transverse
cross-section of the hollow-fiber membrane module or the module
length but is preferably from 70 to 400 m.sup.3/m.sup.2/hr per area
in a transverse cross-section of the hollow-fiber membrane
module.
Second Embodiment
[0182] The configuration of the hollow-fiber membrane module
according to a second embodiment of the present invention is
described by referring to the drawings. FIG. 1 is a schematic
vertical cross-sectional diagram of the hollow-fiber membrane
module according to the second embodiment of the present
invention.
[0183] The hollow-fiber membrane module 100 illustrated in FIG. 1
includes a cylindrical case 3 being open at both ends, a large
number of hollow-fiber membranes 1 housed in the cylindrical case
3, an upper cap 6 attached to the upper part of the cylindrical
case 3, and a lower cap 7 attached to the lower part of the
cylindrical case 3. Furthermore, the hollow-fiber membrane module
100 includes a first potting part 4, a second potting part 5, etc.
Here, the "upper" and "lower" indicate the top and bottom in a
posture when using the module 100 and correspond to the top and
bottom of FIG. 1.
[0184] On a side surface of the cylindrical case 3, a raw liquid
outlet 10 is provided near the upper end of the cylindrical
case.
[0185] The large number of hollow-fiber membranes 1 are bundled to
form a hollow-fiber membrane bundle 2. The filling ratio of the
hollow-fiber membrane bundle 2 in the cylindrical case 3 is
preferably from 41 to 80%. Details of the filling ratio are
described later.
[0186] The first potting part 4 is also referred to as an upper
potting part. The first potting part 4 is formed of an adhesive and
liquid-tightly and airtightly bonds the upper-side end part
(corresponding to the "first end part") of the hollow-fiber
membrane bundle 2 to the cylindrical case 3 while letting an end
face of the hollow-fiber membrane 1 be open. That is, the
hollow-fiber membrane bundles 2 are bundled by the first potting
part 4 and fixed to the inner wall of the cylindrical case 3.
[0187] The hollow-fiber membrane module 100 further includes a flow
regulating cylinder 12. The flow regulating cylinder 12 is a
tubular member disposed inside of the cylindrical case 3. The flow
regulating cylinder 12 is disposed below the first potting part 4.
The top and bottom of the flow regulating cylinder 12 are open, and
an opening, such as a plurality of slits, is provided on a side
surface. The flow regulating cylinder 12 can pass a liquid through
the opening. The flow regulating cylinder 12 is provided on the
periphery of the raw liquid outlet 10 with the purpose of
preventing the treated raw liquid from channeling. For example, in
the case of performing cross-flow filtration with a hollow-fiber
membrane module without a flow regulating cylinder 12, the flow
velocity of the raw liquid within the cylindrical case 3 is
increased on the raw liquid outlet 10 side (left side of FIG. 1)
and reduced on the side of a surface opposing the raw liquid outlet
10 (right side of FIG. 1) and therefore, the hollow-fiber membrane
cleaning performance may be insufficient on the side of a surface
opposing the raw liquid outlet 10 (right side of FIG. 1). When the
flow regulating cylinder 12 is provided, channeling within the
cylindrical case 3 is prevented, and the hollow-fiber membrane
cleaning performance can thereby be enhanced.
[0188] The second potting part 5 is also referred to as a lower
potting part. The second potting part 5 is formed of an adhesive
and in the lower-side end part (corresponding to the "second end
part") of the hollow-fiber membrane bundle 2, is bonded to the
cylindrical case 3 while sealing the lower end face of the
hollow-fiber membrane 1. More specifically, the second potting part
5 is disposed to face the first potting part 4 within the
cylindrical case 3. Thus, in the lower part of the separation
membrane module, the hollow part of the hollow-fiber membrane
bundle 2 is sealed by an adhesive and is in a state incapable of
opening. The hollow-fiber membrane bundles 2 are bundled by the
second potting part 5 and fixed to the inner wall of the
cylindrical case 3.
[0189] The second potting part 5 has a through hole 11 continuing
from a surface opposing the first potting part 4 to the backward
surface. The through hole 11 has a role as a raw liquid passage or
an air passage at the time of air scrubbing. FIG. 2 is an A-A line
cross-sectional view of the hollow-fiber membrane module 100 of
FIG. 1 and illustrates an example of the arrangement of through
holes 11 in the second potting part 5. In order to prevent a raw
liquid channeling during cross-flow filtration or an air channeling
during air scrubbing, the through holes 11 are preferably arranged
evenly in the second potting part.
[0190] The upper cap 6 has a filtered liquid outlet 9. The upper
cap 6 is liquid-tightly and airtightly attached to the upper part
of the cylindrical case 3. The upper cap 6 is attachable/detachable
relative to the upper part of the cylindrical case 3. The lower cap
7 has a raw liquid inflow port 8. The lower cap 7 is liquid-tightly
and airtightly attached to the lower part of the cylindrical case
3. The lower cap 7 is attachable/detachable relative to the lower
part of the cylindrical case 3.
[0191] The raw liquid flows into the hollow-fiber membrane module
100 through the raw liquid inflow port 8 of the lower cap 7, and a
raw liquid having not passed through the hollow-fiber membrane 1 is
discharged from the raw liquid outlet 10 to the outside of the
hollow-fiber membrane module 100. A filtered liquid having passed
through the hollow-fiber membrane 1 is discharged from the filtered
liquid outlet 9 of the upper cap 6 to the outside of the
hollow-fiber membrane module 100. A system of filtering a raw
liquid in this way while flowing it in parallel to the membrane
surface is referred to as cross-flow filtration and has an effect
of preventing suspended substances, etc. in the raw liquid from
depositing on the membrane surface or an effect of preventing
components contained in the raw liquid from causing concentration
polarization on the membrane surface. In addition, a system of, as
in FIG. 1, feeding a raw liquid to the outer side of the
hollow-fiber membrane and performing filtration from the outer side
to the inner side is referred to as an external pressure system.
Conversely, a system of performing filtration from the inner side
to the outer side of the hollow-fiber membrane is referred to as an
internal pressure system.
[0192] In the case of performing cross-flow filtration, when the
membrane surface linear velocity of raw liquid is increased, the
shear stress acting on the membrane surface increases, and the
cleaning performance is enhanced. In the cross-flow filtration, a
raw liquid flows in through the raw liquid inflow port 8 of the
hollow-fiber membrane module 100, and the raw liquid is discharged
from the raw liquid outlet 10. In addition, the filtered liquid is
delivered to the upper part of the hollow-fiber membrane module 100
through the hollow part of the hollow-fiber membrane and discharged
from the filtered liquid outlet 9. The membrane surface linear
velocity of cross-flow filtration is preferably from 0.3 to 5 m/s,
but if the membrane surface linear velocity is increased, the
stress acting on the hollow-fiber membrane increases and therefore,
the hollow-fiber membrane may be broken. Above all, in the case of
an external pressure-type hollow-fiber membrane module 100
illustrated in FIG. 1, the raw liquid flows out from the raw liquid
outlet 10 provided on a side surface of the cylindrical case 3 and
therefore, a raw liquid flow in a direction perpendicular to the
long axis direction of the hollow-fiber membrane is generated near
the raw liquid outlet 10, as a result, a drag force on the
hollow-fiber membrane is produced. The drag force is proportional
to the square of the flow velocity and therefore, when the membrane
surface linear velocity of cross-flow filtration is increased, a
large drag force may be produced on the hollow-fiber membrane
around the raw liquid outlet 10 to cause breakage of the
hollow-fiber membrane. In order to prevent breakage of the
hollow-fiber membrane during cross-flow filtration, the breaking
strength of the hollow-fiber membrane is preferably 25 MPa or more,
more preferably 27 MPa or more.
[0193] Incidentally, a smaller diameter of the hollow-fiber
membrane leads to an increase in the specific surface area and is
advantageous in view of membrane area but poses a problem that the
pressure loss at the time of passing of liquid in the hollow part
increases. Accordingly, the inside diameter of the hollow-fiber
membrane is preferably 0.5 mm or more. In addition, in order to
increase the specific surface area of the hollow-fiber membrane,
the outside diameter of the hollow-fiber membrane is preferably 3.0
mm or less. On the other hand, in the external pressure-type
hollow-fiber membrane module, if the transmembrane pressure
difference is high, the hollow-fiber membrane may be buckled. As
the outside diameter/inside diameter ratio of the hollow-fiber
membrane is larger, the pressure resistance is increased and
buckling is less likely to occur. For this reason, the outside
diameter/inside diameter ratio is preferably 1.5 or more.
[0194] In the cross-flow filtration, the membrane surface is
cleaned by a raw liquid stream flowing in parallel to the membrane
surface, but with the same average linear velocity of raw liquid
within the hollow-fiber membrane module, as the distance between
hollow-fiber membranes is smaller, the shear stress acting on the
membrane surface is higher, and the membrane surface cleaning
effect increases. In order to increase the cleaning effect during
cross-flow filtration by reducing the inter-membrane distance
between hollow-fiber membranes, the filling ratio of the
hollow-fiber membrane within the hollow-fiber membrane module is
preferably from 41 to 80%, more preferably from 50 to 70%. When the
filling ratio of the hollow-fiber membrane is 41% or more, the
distance between membranes is reduced, making it possible to
increase the cleaning efficiency at the time of cross-flow
filtration and prevent a rise in the transmembrane pressure
difference. In addition, as the filling ratio of the hollow-fiber
membrane is higher, the membrane surface linear velocity can be
increased with the same flow rate of raw liquid and thus cleaning
effect can be enhanced. Meanwhile, when the filling ratio of the
hollow-fiber membrane is 80% or less, the hollow-fiber membrane is
easily fixed by the potting part.
[0195] The filling ratio of the hollow-fiber membrane as used
herein indicates the proportion of the area occupied by a
hollow-fiber membrane portion in a transverse cross-section (in
FIG. 1, a plane parallel to the horizontal direction and
perpendicular to the paper plane) of the cylindrical case 3 of the
hollow-fiber membrane module between the first potting part and the
second potting part. Denoting S1 as the cross-sectional area of a
hollow-fiber membrane existing portion on the inner side of the
cylindrical case 3 and S2 as the total cross-sectional area of the
hollow-fiber membrane, the filling ratio of the hollow-fiber
membrane can be represented by the following formula (3). Here, in
the case where a member other than the hollow-fiber membrane, such
as flow regulating cylinder 12, is present, the cross-sectional
area obtained by subtracting the cross-sectional area of the member
other than the hollow-fiber membrane from the cross-sectional area
on the inner side of the cylindrical case 3 is denoted by S. In
addition, the nozzle portion on a side surface of the cylindrical
case 3, which is provided as the raw liquid outlet 10, is also not
included in the cross-sectional area S. When an inner-side member
such as flow regulating cylinder 12, a reduced diameter part or an
expanded diameter part is present in the cylindrical case 3, the
cross-sectional area S is changed in that portion. In the present
invention, with respect to the space between the second potting
part-side interface of the first potting part of the hollow-fiber
membrane module and the first potting-side interface of the second
potting part, the cross-sectional area S is calculated for 10 sites
at regular intervals and denoting the average value thereof as the
cross-sectional area S1 of the hollow-fiber membrane existing
portion, the filling ratio of the hollow-fiber membrane is
calculated according to the following formula (3):
Filling ratio [%] of hollow-fiber membrane=S2/S1.times.100 (3)
[0196] Here, the total cross-sectional area S2 of the hollow-fiber
membrane can be represented by the following formula (4). With
respect to 10 hollow-fiber membranes in the hollow-fiber membrane
module, the outside diameter is measured for every two directions
of longest direction and shortest direction, and the average value
of measured values of a total of 20 sites is designated as the
outside diameter R of the hollow-fiber membrane. Using the outside
diameter R and assuming the hollow-fiber membrane is a perfect
circle, the total cross-sectional area S2 of the hollow-fiber
membrane is calculated according to formula (4):
S2=[circular constant].times.[outside diameter R of hollow-fiber
membrane/2].sup.2.times.[number of hollow-fiber membranes within
hollow-fiber membrane module] (4)
[0197] The above-described average linear velocity of raw liquid
within the hollow-fiber membrane module can be represented by the
following formula (5):
Average linear velocity [m/s]=flow rate of raw liquid
[m.sup.3/s]/(S1-S2)[m.sup.2] (5)
<Potting Method of Hollow-Fiber Membrane Module>
[0198] Bundling hollow-fiber membranes with an adhesive is referred
to as potting. The method for potting includes, as representative
methods, a centrifugal potting method in which a liquid adhesive is
infiltrated among hollow fiber membranes by utilizing centrifugal
force and then cured; and a static potting method in which a liquid
adhesive is fed by a metering pump or head, allowed to naturally
flow and thereby infiltrate among hollow fiber membranes 1, and
then cured. In the centrifugal potting method, an adhesive readily
infiltrates among hollow fiber membranes due to centrifugal force,
and even a high-viscosity adhesive can be used.
<Material of Hollow-Fiber Membrane>
[0199] The material for the hollow-fiber membrane of the
hollow-fiber membrane module of the present invention is not
particularly limited, but a hollow-fiber membrane containing, for
example, a fluororesin-based polymer may be used.
[0200] The fluororesin-based polymer as used in the present
description means a resin containing at least one of a vinylidene
fluoride homopolymer and a vinylidene fluoride copolymer. The
fluororesin-based polymer may contain a plurality of kinds of
vinylidene fluoride copolymers.
[0201] The vinylidene fluoride copolymer is a polymer having a
vinylidene fluoride residue structure and is typically a copolymer
of a vinylidene fluoride monomer and other fluorine-based monomer,
etc. Such a copolymer includes, for example, a copolymer of
vinylidene fluoride and one or more kinds of monomers selected from
vinyl fluoride, tetrafluoroethylene, hexafluoropropylene and
chlorotrifluoroethylene.
[0202] In addition, a monomer other than the above-described
fluorine-based monomer, such as ethylene, may be copolymerized to
the extent not impairing the effects of the present invention.
[0203] The weight average molecular weight of the fluororesin-based
polymer may be appropriately selected according to the strength and
water permeation performance required for the polymer separation
membrane, but as the weight average molecular weight is larger, the
water permeation performance is decreased, and as the weight
average molecular weight is smaller, the strength is decreased. For
this reason, the weight average molecular weight is preferably from
50,000 to 1,000,000. In the case of a water treatment application
where the polymer separation membrane is subject to chemical
cleaning, the weight average molecular weight is preferably from
100,000 to 700,000, more preferably from 150,000 to 600,000.
[0204] The hollow-fiber membrane preferably contains the
fluororesin-based polymer as a main component, and the proportion
of the fluororesin-based polymer in the hollow-fiber membrane is
preferably 80 wt % or more, more preferably 90 wt % or more, still
more preferably 95 wt % or more. The hollow-fiber membrane may be
composed of only the fluororesin-based polymer.
[0205] Here, the "hollow-fiber membrane containing the
fluororesin-based polymer as a main component" can be interchanged
with the "hollow-fiber membrane based on the fluororesin-based
polymer". In the present description, other elements are also
described by the phrase "X contains Y as a main component", and
this can similarly be interchanged with "X is based on Y".
<Orientation of Molecular Chain>
[0206] In the hollow-fiber membrane of the present invention, at
least part of the molecular chain of the fluororesin-based polymer
is oriented in the longitudinal direction of the hollow-fiber
membrane, and the orientation degree .pi. is 0.4 or more and less
than 1.0. The orientation degree t is calculated from a half-width
H (.degree.) obtained by wide-angle X-ray diffraction measurement,
based on the following formula (2):
Orientation degree .pi.=(180.degree.-H)/180.degree. (2)
(wherein H is a half-width (.degree.) of the diffraction intensity
distribution in the circumferential direction of a wide-angle X-ray
diffraction image).
[0207] The orientation of the molecular chain in the longitudinal
direction of the hollow-fiber membrane and the method for measuring
the orientation degree r thereof are specifically described
below.
[0208] In order to calculate the orientation degree .pi., the
hollow-fiber membrane is fixed to a fiber sample stage by arranging
its longitudinal direction to run vertically. Here, the short-side
direction of the hollow-fiber membrane is a direction parallel to
the diameter direction of the hollow fiber, and the longitudinal
direction is a direction perpendicular to the short-side
direction.
[0209] When X-ray diffraction is performed, an annular diffraction
image called a Debye-Scherrer ring is obtained. In the case of a
non-oriented sample, a great change is not observed in the
diffraction intensity along the Debye-Scherrer ring, but in the
case of an oriented sample, the intensity distribution is deviated
on the Debye-Scherrer ring. Accordingly, the orientation degree can
be calculated from this intensity distribution based on formula
(2).
[0210] More specifically, in the case where the molecular chain is
non-oriented, when 2.theta./.theta. scanning is performed in the
short-side direction (i.e., when a diffraction pattern showing a
diffraction intensity distribution in the diameter direction of
Debye-Scherrer ring is obtained), a peak is observed at a position
around the diffraction angle 2.theta.=20.degree.. The abscissa axis
of the diffraction pattern obtained here is the diffraction angle
2.theta. of X-ray, and the ordinate axis is the diffraction
intensity. Furthermore, when the sample is scanned in the azimuth
angle .beta. direction by fixing the diffraction angle 2.theta. to
the peak position above, i.e., around 20.degree., a diffraction
pattern in which the abscissa axis shows the azimuth angle .beta.
and the ordinate axis shows the diffraction intensity (i.e., a
diffraction intensity distribution along the circumferential
direction of Debye-Scherrer ring at the position of diffraction
angle 2.theta.=20.degree.) is obtained. In the case of a
non-oriented sample, the diffraction intensity is substantially
constant throughout 360.degree. in the circumferential direction of
Debye-Scherrer ring.
[0211] On the other hand, in the case where the molecular chain is
oriented in the longitudinal direction of the hollow-fiber
membrane, a strong diffraction intensity is observed on the azimuth
angle corresponding to the short-side direction of the hollow-fiber
membrane (i.e., on the equatorial line) on the Debye-Scherrer ring
around 2.theta.=20.degree., and a small diffraction intensity is
obtained in other portions. More specifically, in the case of an
oriented sample, the diffraction intensity distribution in the
diameter direction of Debye-Scherrer ring shows a diffraction peak
around 2.theta.=20.degree., similarly to a non-oriented sample, and
the distribution in the circumferential direction shows, unlike a
non-oriented sample, a diffraction peak on the azimuth angle
corresponding to the short-side direction of the hollow-fiber
membrane. For example, FIG. 7 is a diagram illustrating the
intensity distribution in the azimuth angle direction at
2.theta.=20.4.degree. of the hollow-fiber membrane of Example 11
(Reference Example 8), and in this diagram, a peak is observed
around .beta.=90.degree. and 270.degree..
[0212] In the description above, the position of diffraction peak
in the diameter direction of Debye-Scherrer ring (i.e., the value
of 2.theta. corresponding to the diffraction peak) is "around
20.degree.". However, the value of 2.theta. differs depending on
the structure or blending of polymer and may range from 15 to 250.
For example, when X-ray diffraction is performed for a
polyvinylidene fluoride homopolymer having an .alpha. crystal or
.beta. crystal, a diffraction peak derived from a (110) plane of
.alpha. crystal or .beta. crystal, i.e., a plane parallel to
molecular chain, is observed around 2.theta.=20.4.degree..
[0213] As described above, the intensity distribution in the
azimuth angle direction is obtained by fixing the value of
diffraction angle 2.theta. and furthermore, measuring the intensity
in the range from 0.degree. up to 360.degree. in the azimuth angle
direction (circumferential direction). This intensity distribution
may also be said to be an intensity distribution obtained by
scanning a crystal peak on a diffraction image in the
circumferential direction. Here, when the ratio between the
intensity at an azimuth angle of 180.degree. (longitudinal
direction) and the intensity at an azimuth angle of 90.degree.
(short-side direction) is 0.80 or less or 1.25 or more, it is
regarded that a peak is present, and using the intensity
distribution in this azimuth angle direction, the width at a
position of half the peak height (half-width H) is determined.
[0214] The orientation degree .pi. is calculated by substituting
the half-width H into formula (2).
[0215] In the hollow-fiber membrane of the present invention, the
orientation degree .pi. of the molecular chain in the longitudinal
direction of the hollow-fiber membrane is 0.4 or more and less than
1.0, preferably 0.5 or more and less than 1.0, more preferably 0.6
or more and less than 1.0. When the orientation degree .pi. is 0.4
or more, the mechanical strength of the hollow-fiber membrane is
increased. Incidentally, when wide-angle X-ray diffraction
measurement is performed at measurement points at intervals of 1 cm
in the longitudinal direction of the hollow-fiber membrane, it is
preferred that at 80% or more of the measurement points, the
orientation degree .pi. is 0.4 or more and less than 1.0.
[0216] In the intensity distribution obtained by scanning a crystal
peak in the circumferential direction, when the ratio between the
intensity at an azimuth angle of 180.degree. and the intensity at
an azimuth angle of 90.degree. is more than 0.80 and less than
1.25, it is regarded that a peak is absent. That is, in this case,
the fluororesin-based polymer is determined to be non-oriented.
[0217] In the case where the hollow-fiber membrane contains an
.alpha. crystal or .beta. crystal of polyvinylidene fluoride, the
half-width H is preferably determined from an intensity
distribution obtained by circumferentially scanning a crystal peak
(2.theta.=20.4.degree.) derived from a (110) plane of the .alpha.
crystal or .beta. crystal of polyvinylidene fluoride in wide-angle
X-ray diffraction measurement.
[0218] The orientation of the molecular chain in the hollow-fiber
membrane can also be determined by orientation analysis according
to Raman spectroscopy. First, a hollow-fiber membrane is sliced by
cutting with a microtome from a cross-section along the
longitudinal direction of the hollow-fiber membrane. The
thus-obtained section is observed under an optical microscope, and
laser Raman measurement is thereby performed at 1 .mu.m intervals
along the longitudinal direction of a columnar texture while
checking the columnar texture. The number of measurement points in
one columnar texture is a value obtained by dividing the
longitudinal length (.mu.m) of the later-described columnar texture
by 1 .mu.m (rounded down to the nearest integer). For example, when
the longitudinal length of the columnar texture is 20.5 .mu.m, the
number of measurement points is 20.
[0219] For example, in the case where the fluororesin-based polymer
is a polyvinylidene fluoride homopolymer, the Raman band around
1,270 cm.sup.-1 is assigned to a coupling mode of CF.sub.2
(fluorocarbon) stretching vibration and CC (carbon-carbon)
stretching vibration. The vibration direction of these vibrations
is in a mode parallel to molecular chain. Meanwhile, the vibration
direction of the Raman band around 840 cm.sup.-1 is perpendicular
to molecular chain. Since strong Raman scattering is obtained when
the vibration direction of molecular chain coincides with the
polarization direction of incident light, the scattering intensity
ratio of these vibration modes is changed in correlation with the
orientation degree.
[0220] The Raman orientation parameter can therefore be calculated
according to the following formula (1). The Raman orientation
parameter shows a larger value as the orientation in the
longitudinal direction of the hollow-fiber membrane is higher,
shows a value of 1 when non-oriented, and shows a value smaller
than 1 when the orientation in the short-side direction is
high.
Raman orientation parameter=(I1270/I840) parallel/(I1270/I840)
perpendicular (1)
[0221] In formula (1),
[0222] parallel condition: the longitudinal direction of the
hollow-fiber membrane is parallel to the polarization
direction,
[0223] perpendicular condition: the longitudinal direction of the
hollow-fiber membrane is orthogonal to the polarization
direction,
[0224] I1270 parallel: the intensity of Raman band at 1,270
cm.sup.-1 under parallel condition,
[0225] I1270 perpendicular: the intensity of Raman band at 1,270
cm.sup.-1 under perpendicular condition,
[0226] I840 parallel: the intensity of Raman band at 840 cm.sup.-1
under parallel condition, and
[0227] I840 perpendicular: the intensity of Raman band at 840
cm.sup.-1 under perpendicular condition.
[0228] In one hollow-fiber membrane, 10 columnar textures different
from each other, having a length of 0.5 to 1.5 times the
representative value of the longitudinal length of the
later-described columnar texture, are selected. With respect to
each columnar texture, laser Raman measurement is performed, and
the Raman orientation parameters of respective measurement points
are calculated according to formula (1). An average value of the
obtained values is defined as the Raman orientation parameter .nu..
In addition, an operation of selecting a largest Raman orientation
parameter and a smallest Raman orientation parameter among the
measurement points of one columnar texture is performed for 10
columnar textures different from each other. With respect to
selected 10 largest Raman orientation parameters and 10 smallest
Raman orientation parameters, respective average values are
determined and taken as a maximum Raman orientation parameter M and
a minimum Raman orientation parameter m, and MIm is calculated. In
order to accurately obtain the Raman orientation parameter .nu.,
maximum Raman orientation parameter M, minimum Raman orientation
parameter m and M/m, the measurement is preferably performed for 20
columnar textures different from each other.
[0229] In the hollow-fiber membrane of the present invention, the
Raman orientation parameter .nu. of the molecular chain in the
longitudinal direction of the hollow-fiber membrane is preferably
3.0 or more, more preferably 3.4 or more, still more preferably 3.7
or more. When the Raman orientation parameter .nu. is 3.0 or more,
the strength of the hollow-fiber membrane is increased.
[0230] It is considered that the maximum Raman orientation
parameter M and the minimum Raman orientation parameter m indicate
respectively a main orientation site in the columnar texture and a
point of effort during stretching. Accordingly, M and m may be set
to appropriate ranges by taking into account a balance of
performances of the obtained hollow-fiber membrane, such as
strength, elongation and water permeability. M/m is preferably
larger because of a tendency that orientation of the molecular
chain develops and the strength of the hollow-fiber membrane
increases. For this reason, in the present invention, M/m is
preferably 3 or more, more preferably 4 or more, still more
preferably 5 or more.
[0231] There is a tendency that the orientation degree .pi.
determined by wide-angle X-ray diffraction measurement represents
the orientation of molecular chain of the entire porous
hollow-fiber membrane and the Raman orientation parameter .nu.
determined by Raman spectroscopy represents the orientation of
molecular chain when focus is directed onto the columnar texture of
the porous hollow-fiber membrane, i.e., the orientation of local
molecular chain. When both the entire and local molecular chains of
the porous hollow-fiber membrane are strongly oriented, the
strength of the hollow-fiber membrane increases. For this reason,
it is preferred that the orientation degree .pi. is 0.6 or more and
less than 1.0 and the Raman orientation parameter .nu. is 3.4 or
more, and it is more preferred that the orientation degree .pi. is
0.7 or more and less than 1.0 and the Raman orientation parameter
.nu. is 3.7 or more.
[0232] As a specific configuration, in the hollow-fiber membrane,
the molecular chain of the fluororesin-based polymer is preferably
oriented in the longitudinal direction of the hollow-fiber
membrane.
<Columnar Texture>
(a) Dimension
[0233] The hollow-fiber membrane has a columnar texture oriented in
the longitudinal direction of the hollow-fiber membrane. The
"columnar texture" is a solid material having a uniform thickness
and having a shape long in one direction. The aspect ratio
(longitudinal length/short-side length) of the columnar texture is
preferably 3 or more.
[0234] Here, the "longitudinal length" indicates a length in the
longitudinal direction of the columnar texture. The "short-side
length" is an average length in the short-side direction of the
columnar texture. Furthermore, "oriented in the longitudinal
direction" means that out of angles between the longitudinal
direction of the columnar texture and the longitudinal direction of
the hollow-fiber membrane, the acute angle is within
20.degree..
[0235] The longitudinal length and short-side length can be
measured as follows. A hollow-fiber membrane is cut along the
longitudinal direction of the hollow-fiber membrane, and the
obtained cross-section is observed using a scanning electron
microscope (SEM). The magnification is variable according to the
length of the columnar texture and is set to a level allowing a
visual field to include the entire figure of each of 5, preferably
10, columnar textures over its longitudinal direction. In the case
where the length in the longitudinal direction length varies in one
columnar texture, a maximum length in the longitudinal direction
may be measured as the longitudinal length. The short-side length
is determined by measuring the length in each short-side direction
at a predetermined number of arbitrary measurement points in one
columnar texture and calculating an average value thereof. The
number of measurement points is a value obtained by dividing the
longitudinal length (m) by 1 .mu.m (rounded down to the nearest
integer). For example, when the longitudinal length of the columnar
texture is 20.5 .mu.m, the number of measurement points is 20. In
this connection, when the value becomes 21 or more, the length may
be measured at arbitrary 20 points.
[0236] The longitudinal length of the columnar texture is not
particularly limited but is preferably 7 .mu.m or more, more
preferably 10 .mu.m or more, still more preferably 15 .mu.m or
more. The longitudinal length of the columnar texture is, for
example, preferably 50 .mu.m or less, more preferably 40 .mu.m or
less.
[0237] In the present invention, the short-side length of the
columnar texture is preferably from 0.5 to 3 .mu.m. The short-side
length is preferably in the range above, because high strength
performance and high pure-water permeation performance are
obtained. When the short-side length of the columnar texture is 0.5
.mu.m or more, physical strength of the columnar texture itself is
increased and therefore, high strength is obtained. When the
short-side length of the columnar texture is 3 .mu.m or less, the
void among columnar textures becomes large and in turn, good
pure-water permeation performance is obtained. The short-side
length of the columnar texture is more preferably from 0.7 to 2.5
.mu.m, still more preferably from 1 to 2 .mu.m.
[0238] In the hollow-fiber membrane of the present invention,
preferable ranges of representative values of the longitudinal
length and short-side length of the columnar texture are
respectively the same as the above-described preferable ranges of
the longitudinal length and short-side length of each individual
columnar texture. In addition, as for the effects due to each
representative value being in that range, description of effects
when individual columnar textures have a dimension in that range is
applied.
[0239] The representative value of the longitudinal length is
measured as follows. Similarly to the measurement of the
longitudinal length, the longitudinal length is measured at 3
sites, preferably 5 sites, in the hollow-fiber membrane for 5,
preferably 10, columnar textures per site. With respect to the
obtained values of the longitudinal length, an average value is
determined and can be used as the representative value of the
longitudinal length of the columnar texture.
[0240] The representative value of the short-side length is
determined by measuring the short-side length (calculated as an
average value) as described above for columnar textures which were
subject to measurement of the representative value of the
longitudinal length, and calculating an average value thereof.
[0241] In the hollow-fiber membrane of the present invention, the
representative value of the aspect ratio of the columnar texture
calculated from the representative value of the longitudinal length
and the representative value of the short-side length is preferably
3 or more, more preferably 5 or more, still more preferably 10 or
more, yet still more preferably 20 or more.
[0242] In the present invention, it is preferred that the
short-side length of the columnar texture is from 0.5 to 3 .mu.m
and the aspect ratio of the columnar texture is 3 or more.
Incidentally, the upper limit of the aspect ratio is not
particularly limited but may be, for example, 50 in consideration
of the existing production method, etc. of the hollow-fiber
membrane.
(b) Thickness Uniformity
[0243] As described later, the hollow-fiber membrane of the present
invention can be produced by forming a hollow fiber from a
membrane-forming solution containing a polymer, and stretching the
hollow fiber. For the sake of convenience, the state before
stretching is referred to as "hollow fiber", and the state after
stretching is referred to as "hollow-fiber membrane".
[0244] The thickness uniformity (the later-described average value
D) of the columnar texture in the hollow-fiber membrane after
stretching is preferably 0.60 or more, more preferably 0.70 or
more, still more preferably 0.80 or more, yet still more preferably
0.90 or more. Although the thickness uniformity is 1.0 at a
maximum, the columnar texture may have a thickness uniformity of
less than 1.0.
[0245] In the hollow-fiber membrane, the columnar texture has a
high thickness uniformity in this way, i.e., a narrowed portion is
little formed in the columnar texture, and the elongation of the
hollow-fiber membrane is thereby increased.
[0246] When the hollow-fiber membrane after stretching keeps high
elongation, this is advantageous in that fiber breakage is less
likely to occur even when a load is abruptly applied. The
elongation at break of the hollow-fiber membrane is preferably 50%
or more, more preferably 80% or more. The upper limit of the
elongation at break of the hollow-fiber membrane is not
particularly limited but is, for example, 500% in consideration of
the thickness uniformity above.
[0247] The thickness uniformity is described below. As the length
variation among respective short-side directions of the columnar
texture is smaller, a narrowed portion is less formed in the
columnar texture, resulting in high thickness uniformity, and the
columnar texture comes close to a perfect column.
[0248] The thickness uniformity of the columnar texture is
determined by comparing a first cross-section and a second
cross-section each running in parallel to the short-side direction
of the hollow-fiber membrane. This is specifically described
below.
[0249] At the beginning, a first cross-section and a second
cross-section running in parallel to each other are selected. The
distance between the first cross-section and the second cross
section is set to be 5 .mu.m. First, a portion composed of resin
and a void portion are distinguished in each cross-section, and the
area of resin portion and the area of void portion are measured.
Next, the area of a portion where when the first cross-section is
projected onto the second cross-section, the portion composed of
resin in the first cross-section and the portion composed of resin
in the second cross-section are overlapped, namely, the overlap
area, is determined. With respect to arbitrary 20 pairs of first
cross-section and second cross-section in one hollow-fiber
membrane, thickness uniformities A and B are determined based on
the following formulae (6) and (7), respectively:
Thickness uniformity A=(overlap area)/(area of resin portion of
second cross-section) (6)
Thickness uniformity B=(overlap area)/(area of resin portion of
first cross-section) (7)
[0250] That is, 20 pairs of thickness uniformities A and B are
obtained for one hollow-fiber membrane. A larger value means that
the thickness of the columnar texture is more uniform. Then, with
respect to each pair, an average value C of thickness uniformities
A and B is calculated. That is, 20 average values C are obtained
for one hollow-fiber membrane. With respect to these average values
C, an average value D is further calculated. The average value D is
the thickness uniformity of this hollow-fiber membrane.
[0251] In the case where 80% or more of 20 average values C
calculated for one hollow-fiber membrane have a value of 0.60 or
more, the hollow-fiber membrane can be said to have the columnar
texture.
[0252] In measuring the thickness uniformity, in order to clearly
distinguish the resin portion and the void portion, it is
preferable to previously perform resin-embedding of the
hollow-fiber membrane in an epoxy resin, etc. and staining
treatment of the epoxy resin, etc. with, for example, osmium. By
such resin embedding/staining treatment, the void portion is filled
with an epoxy resin, etc., and at the time of the later-described
cross-sectional processing with a focused ion beam, the portion
composed of a fluororesin-based polymer and the void portion (i.e.,
the epoxy resin portion) can be clearly distinguished, as a result,
high observation accuracy is obtained.
[0253] Furthermore, in order to obtain the above-described first
cross-section and second cross-section each running in parallel to
the short-side direction of the hollow-fiber membrane, a scanning
electron microscope (SEM) equipped with a focused ion beam (FIB) is
preferably used. A face parallel to the short-side direction of the
hollow-fiber membrane is cut out using FIB, and FIB cutting and SEM
observation are repeatedly conducted 200 times at 50 nm intervals
toward the longitudinal direction of the hollow-fiber membrane. By
such continuous cross-sectional observation, information at a depth
of 10 .mu.m can be obtained. Arbitrary first and second
cross-sections forming faces running in parallel to each other and
being spaced 5 .mu.m apart are selected therefrom, and the
thickness uniformities can be determined using formulae (6) and
(7). The observation magnification may be sufficient if it is a
magnification enabling clear identification of a columnar texture
and a spherical texture, and a magnification of, for example, from
1,000 to 5.000 times may be used.
(c) Composition
[0254] The columnar texture preferably contains the
fluororesin-based polymer as a main component, and the proportion
of the fluororesin-based polymer in the columnar texture is
preferably 80 wt % or more, more preferably 90 wt % or more, still
more preferably 95 wt % or more. The columnar texture may be
composed of only the fluororesin-based polymer.
[0255] In other words, the hollow-fiber membrane has a solid matter
containing a fluororesin-based polymer, and at least part of the
solid matter constitutes a columnar texture. All of solid matters
containing a fluororesin-based polymer may constitute a columnar
texture, or part thereof may have a shape not falling under a
columnar texture. In the hollow-fiber membrane, out of solid
matters containing a fluororesin-based polymer, the proportion of
the solid matter constituting a columnar texture is preferably 80
wt % or more, more preferably 90 w % or more, still more preferably
95 wt % or more.
(d) Columnar Texture in Hollow-Fiber Membrane
[0256] In the hollow-fiber membrane, the principal structure is
preferably a columnar texture. The proportion of the columnar
texture in the hollow-fiber membrane is preferably 80 wt % or more,
more preferably 90 wt % or more, still more preferably 95 wt % or
more. The hollow-fiber membrane may be composed of only a columnar
texture.
[0257] More specifically, the hollow-fiber membrane preferably has,
as the principal structure, a columnar texture containing a
fluororesin-based polymer as a main component. The hollow-fiber
membrane can also be phrased as an assembly of columnar
textures.
<Porosity>
[0258] In the hollow-fiber membrane of the present invention, in
order to satisfy both high pure-water permeation performance and
high strength, the porosity is preferably from 41 to 90%, more
preferably from 50 to 80%, still more preferably from 50 to 70%. If
the porosity is less than 41%, the pure-water permeation
performance is reduced, whereas if it exceeds 90%, the strength
significantly decreases and therefore, the membrane lacks
suitability as a hollow-fiber membrane for water treatment. The
porosity of the hollow-fiber membrane is determined according to
the following formula (8) by using the area of resin portion and
the area of void portion in the above-described cross-section. In
order to increase the accuracy, it is preferable to determine the
porosity for arbitrary 20 or more, preferably 30 or more,
cross-sections and use an average value thereof.
Porosity (%)={100.times.(area of void portion)}/{(area of resin
portion)+(area of void portion)} (8)
<Others>
[0259] The hollow-fiber membrane of the present invention may
contain a texture other than the above-described columnar texture
to the extent not departing from the object of the present
invention. The structure other than the columnar texture includes,
for example, a spherical texture having an aspect ratio
(longitudinal length/short-side length) of less than 3. The
short-side length and longitudinal length of the spherical texture
are preferably from 0.5 to 3 .mu.m. In the case of using a
spherical texture, as long as the short-side length and
longitudinal length thereof are in the range above, reduction in
the strength of the hollow-fiber membrane can be prevented, and
good pure-water permeation performance can be maintained.
[0260] However, if the proportion of such a spherical texture
having an aspect ratio of less than 3 in the hollow-fiber membrane
is increased, there arises a tendency that spherical textures are
increasingly coupled with each other to increase the narrowed
portion and it is difficult to perform high-ratio stretching or
keep the elongation after stretching. For this reason, a smaller
proportion of the spherical texture in the hollow-fiber membrane is
more preferred, and the proportion is preferably less than 20%,
more preferably less than 10%, still more preferably less than 1%,
i.e., almost nil. It is most preferred that the spherical texture
is not present at all.
[0261] Here, the occupancy (%) of each texture is determined
according to the following formula (9) after taking a photograph of
a cross-section in the longitudinal direction of the hollow-fiber
membrane by means of SEM, etc. at a magnification enabling clear
identification of a columnar texture and a spherical texture,
preferably at a magnification of 1,000 to 5,000 times. In order to
increase the accuracy, it is preferable to determine the occupancy
for arbitrary 20 or more, preferably 30 or more, cross-sections and
calculate an average value thereof.
Occupancy (%)={(area occupied by each texture)/(area of entire
photograph)}.times.100 (9)
[0262] Incidentally, the area of the entire photograph and the area
occupied by a texture can be determined preferably by employing,
for example, a method of converting the area into a weight
corresponding to each texture photographed. That is, the photograph
taken may be printed on paper, and the weight of paper
corresponding to the entire photograph and the weight of paper
corresponding to a texture portion cut out therefrom may be
measured. In addition, before taking a photograph by SEM, etc., the
above-described resin embedding/staining treatment and FIB cutting
are preferably applied, because the observation accuracy
increases.
[0263] The hollow-fiber membrane of the present invention may be a
membrane in which a layer having the above-described columnar
texture and a layer having other structure are stacked to the
extent not departing from the object of the present invention.
However, if the thickness of the layer having other structure is
large compared with the layer having the columnar texture, the
object and effects of the present invention can hardly be exerted
and therefore, the ratio of the thickness of the layer having other
structure to the thickness of the layer having the columnar texture
is preferably 0.3 or less, more preferably 0.2 or less.
<Production Method of Hollow-Fiber Membrane>
[0264] The method for producing the hollow-fiber membrane of the
present invention is described below by way of example. The method
for producing a hollow-fiber membrane includes at least:
[0265] 1) a step of forming a hollow fiber having a columnar
texture from a membrane-forming solution containing a
fluororesin-based polymer by thermally induced phase separation, in
which the columnar texture is oriented in the longitudinal
direction and has a thickness uniformity of 0.60 or more and less
than 1.00: and
[0266] 2) a step of stretching the porous hollow fiber obtained in
1) above to 2.0 to 5.0 times in the longitudinal direction.
(a) Preparation of Membrane-Forming Solution
[0267] The production method of the porous hollow-fiber membrane in
the present invention further includes a step of preparing a
fluororesin-based polymer solution. A fluororesin-based polymer
solution (i.e., a membrane-forming solution containing a
fluororesin-based polymer) is prepared by dissolving a
fluororesin-based polymer in a poor or good solvent for the
fluororesin-based polymer at a relatively high temperature of not
less than the crystallization temperature.
[0268] When the polymer concentration in the membrane-forming
solution is high, a hollow-fiber membrane having high strength is
obtained. On the other hand, when the polymer concentration is low,
the porosity of the hollow-fiber membrane is increased, and the
pure-water permeation performance is enhanced. Accordingly, the
concentration of the fluororesin-based polymer is preferably from
20 to 60 wt %, more preferably from 30 to 50 wt %.
[0269] In the present invention, the poor solvent is a solvent in
which the fluororesin-based polymer cannot be dissolved to a
concentration of 5 wt % or more at a low temperature of 60.degree.
C. or less but can be dissolved to a concentration of 5 wt % or
more in a high-temperature region between 60.degree. C. or more and
not more than the melting point of the fluororesin-based polymer
(for example, when the polymer is composed of a vinylidene fluoride
homopolymer alone, about 178.degree. C.). The good solvent is a
solvent in which the fluororesin-based polymer can be dissolved to
a concentration of 5 wt % or more even in a low-temperature region
of 60.degree. C. or less. The nonsolvent is defined as a solvent in
which the fluororesin-based polymer is neither dissolved nor
swollen at a temperature up to the melting point of the
fluororesin-based polymer or the boiling point of the solvent.
[0270] The poor solvent for the fluororesin-based polymer includes
cyclohexanone, isophorone, y-butyrolactone, methyl isoamyl ketone,
propylene carbonate, dimethylsulfoxide, etc., and a mixed solvent
thereof. The good solvent includes N-methyl-2-pyrrolidone,
dimethylacetamide, dimethylformamide, methyl ethyl ketone, acetone,
tetrahydrofuran, tetramethylurea, trimethyl phosphate, etc., and a
mixed solvent thereof. The nonsolvent includes water, hexane,
pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride,
o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene
glycol, triethylene glycol, propylene glycol, butylene glycol,
pentanediol, hexanediol, an aliphatic hydrocarbon such as
low-molecular-weight polyethylene glycol, an aromatic hydrocarbon,
an aliphatic polyhydric alcohol, an aromatic polyhydric alcohol, a
chlorinated hydrocarbon, other chlorinated organic liquids, a mixed
solvent thereof, etc.
(b) Formation of Hollow Fiber
[0271] In the hollow fiber forming step, a hollow fiber is obtained
from a membrane-forming solution containing a fluororesin-based
polymer by utilizing a thermally induced phase separation method of
inducing phase separation by temperature change. In order to
perform the later-described high-ratio stretching of 2.0 times or
more, it is preferred that the hollow fiber has a columnar texture
oriented in its longitudinal direction and the thickness uniformity
of the columnar texture is 0.60 or more and less than 1.00. The
lower limit of the thickness uniformity of the columnar texture is
more preferably 0.70 or more, still more preferably 0.80 or more,
yet still more preferably 0.90 or more.
[0272] In the thermally induced phase separation method, two kinds
of phase separation mechanisms are mainly utilized. One is a
liquid-liquid phase separation method in which a polymer solution
dissolved uniformly at a high temperature is separated into a
polymer-rich phase and a polymer-poor phase due to reduction in the
dissolving ability of the solution during a temperature drop and
the structure is thereafter fixed by crystallization. Another is a
solid-liquid phase separation method in which a polymer solution
dissolved uniformly at a high temperature is phase-separated into a
polymer solid phase and a solvent phase due to occurrence of
crystallization of the polymer during a temperature drop.
[0273] A three-dimensional network structure is mainly formed in
the former method, and a spherical structure constituted by a
spherical texture is mainly formed in the latter method. In the
production of the hollow-fiber membrane of the present invention,
the latter phase separation mechanism is preferably utilized.
Accordingly, a polymer concentration and a solvent, inducing
solid-liquid phase separation, are selected. In the former phase
separation mechanism, it is difficult to develop the
above-described columnar texture oriented in the longitudinal
direction of the hollow fiber membrane. This is because the
polymer-rich phase forms a very fine phase by phase separation
before the structure is fixed, and cannot be made columnar.
[0274] As a specific method, a hollow part-forming liquid is
discharged through an inner tube of a double tube-type spinneret
for spinning of a hollow-fiber membrane while discharging the
above-described membrane-forming solution through an outer tube of
the double tube-type spinneret. The thus-discharged
membrane-forming solution is cooled and solidified in a cooling
bath to obtain a hollow-fiber membrane.
[0275] The fluororesin-based polymer solution is, before being
discharged from the spinneret, held in a specific temperature
condition for a given time under pressure. The pressure is
preferably 0.5 MPa or more, more preferably 1.0 MPa or more. The
temperature T of the polymer solution preferably satisfies
Tc+35.degree. C..ltoreq.T.ltoreq.Tc+60.degree. C., more preferably
satisfies Tc+40.degree. C..ltoreq.T.ltoreq.Tc+55.degree. C. Tc is
the crystallization temperature of the fluororesin-based polymer
solution. The time for which the polymer solution is held under
these pressure and temperature is preferably 10 seconds or more,
more preferably 20 second or more.
[0276] Specifically, at any site of a solution feed line for
delivering the polymer solution to the spinneret, a retention part
for allowing the polymer solution to stay is provided, and a
pressurizing unit for applying a pressure to the retained polymer
solution and a temperature-adjusting unit for adjusting the
temperature of the retained polymer solution (for example, a
heating unit) are provided. The pressurizing unit is not
particularly limited, but by disposing two or more pumps in the
solution feed line, a pressure can be applied to any site
therebetween. The pump includes a piston pump, a plunger pump, a
diaphragm pump, a wing pump, a gear pump, a rotary pump, a screw
pump, etc., and two or more kinds of pumps may be used.
[0277] Through this step, a pressure is applied under the
conditions in which crystallization easily takes place, and
therefore, it is presumed that crystal growth has anisotropy and in
turn, not an isotropic spherical structure but a texture oriented
in the longitudinal direction of the porous hollow fiber membrane
is developed, as a result, a columnar structure is obtained.
[0278] Here, the crystallization temperature Tc of the
fluororesin-based polymer solution is defined as follows. In an
apparatus for differential scanning calorimetry (DSC measurement),
a mixture having the same composition as the composition of the
membrane-forming polymer solution containing a fluororesin-based
polymer, a solvent, etc. is sealed in a sealing type DSC container
and uniformly dissolved by raising the temperature to a dissolution
temperature at a temperature rise rate of 10.degree. C./min and
holding the temperature for 30 minutes, and a rise temperature of a
crystallization peak observed in the process of thereafter lowering
the temperature at a temperature drop rate of 10.degree. C./min is
Tc.
[0279] The cooling bath for cooling the fluororesin-based polymer
solution discharged from the spinneret is described below. In the
cooling bath, a mixed liquid including a poor or good solvent in a
concentration of 50 to 95 wt % and a nonsolvent in a concentration
of 5 to 50 wt % is preferably used. As the poor solvent, use of the
same poor solvent as that in the polymer solution is preferably
employed. For the hollow part-forming liquid, as with the cooling
bath, a mixed liquid including a poor or good solvent in a
concentration of 50 to 95 wt % and a nonsolvent in a concentration
of 5 to 50 wt % is preferably used. As the poor solvent, use of the
same poor solvent as that in the polymer solution is preferably
employed.
[0280] Here, in order to develop not a fibrous texture having a
large number of narrowed portions but a columnar texture having a
uniform thickness, it is preferable to promote polymer
uptake/growth into the narrowed portion. The present inventors have
found that the polymer uptake/growth into the narrowed portion
leads to disappearance of a narrowed portion having high interface
energy and can energetically stabilize and therefore be caused to
preferentially occur over the growth in portions other than the
narrowed portion, and intensive studies have been made on the
method for enhancing the thickness uniformity.
[0281] As a result, it has been found that as the method for
promoting polymer uptake/growth into the narrowed portion, the
thermally induced phase separation preferably includes at least one
of the following cooling steps a) and b):
[0282] a) a step of soaking the membrane-forming solution in a
cooling bath at a temperature Tb satisfying Tc-30.degree.
C.<Tb.ltoreq.Tc: and
[0283] b) a step of soaking the membrane-forming solution in a
cooling bath at a temperature Tb1 satisfying
Tb1.ltoreq.Tc-30.degree. C., followed by soaking in a cooling bath
at a temperature Tb2 satisfying Tc-30.degree.
C.<Tb2.ltoreq.Tc,
(wherein Tc is the crystallization temperature of the
membrane-forming solution containing a fluororesin-based
polymer).
[0284] In the present invention, it has been found that as the
method a), when cooling/solidification in a cooling bath is
performed near the crystallization temperature of the polymer
solution, the cooling/solidification slowly proceeds. In this case,
denoting Tc as the crystallization temperature of the
fluororesin-based polymer solution, the temperature Tb of the
cooling bath is set to satisfy Tc-30.degree. C.<Tb.ltoreq.Tc,
and Tc-20.degree. C.<Tb.ltoreq.Tc is more preferred.
[0285] The passing time in the cooling bath (i.e., soaking time in
the cooling bath) is not particularly limited as long as enough
time to complete the thermally induced phase separation including
polymer uptake/growth into the narrowed portion can be ensured, and
the passing time may be experimentally determined by taking into
account the number of hollow-fiber membranes, the spinning speed,
the bath ratio, the cooling capacity, etc.
[0286] However, in order to achieve thickness uniformity, the
passing time is preferably set to be as long as possible in the
above-described temperature range of the cooling bath and may be,
for example, 10 seconds or more, preferably 20 seconds or more,
more preferably 30 seconds or more.
[0287] In addition, as the method b), two or more stages of cooling
may be performed. Specifically, the cooling step may include a step
of cooling the solution by using a first cooling bath for promoting
generation/growth of a crystal nucleus by increasing the
supercooling degree, and a step of thereafter cooling the solution
by using a second cooling bath for promoting polymer uptake/growth
into the narrowed portion. The cooling step by the second cooling
bath utilizes a phenomenon that the polymer uptake/growth into the
narrowed portion preferentially occurs mainly in the structure
coarsening process of the phase separation.
[0288] In this case, when the temperature Tb1 of the first cooling
bath for cooling the fluororesin polymer solution discharged from
the spinneret satisfies Tb1.ltoreq.Tc-30.degree. C., the generation
and growth of a crystal nucleus can be promoted by increasing the
supercooling degree, and when the temperature Tb2 of the second
cooling bath is set to a temperature near the crystallization
temperature (specifically, set to satisfy Tc-30.degree.
C.<Tb2.ltoreq.Tc, preferably Tc-20.degree. C.<Tb2.ltoreq.Tc),
the polymer uptake/growth into the narrowed portion can be
promoted. Tc is the crystallization temperature of the polymer
solution.
[0289] The passing time in each cooling bath can be varied, but it
is favorable to set, for example, the passing time in the first
cooling bath to be from 1 to 20 seconds, preferably from 3 to 15
seconds, more preferably from 5 to 10 seconds, and set the passing
time in the second cooling bath to be 10 seconds or more,
preferably 20 seconds or more, more preferably 30 seconds or
more.
[0290] When a texture having a thickness uniformity of less than
0.60 is referred to as "fibrous texture" so as to distinguish it
from the columnar texture, the hollow-fiber membrane disclosed in
JP-A-2006-297383 (Patent Document 5) is a hollow-fiber membrane
having a fibrous texture. Such a porous hollow-fiber membrane
having a fibrous texture is relatively excellent in strength and
pure-water permeation performance, and the present inventors have
therefore attempted to increase the strength by stretching this
membrane. However, it has been found that the membrane cannot be
uniformly stretched and the strength cannot be increased.
[0291] In general, a porous membrane used for water treatment has a
large number of void parts for passing water and since destruction
of the texture proceeds starting from a void part at the time of
stretching, the stretching itself is very difficult. This tendency
is prominent in particular when the hollow-fiber membrane has a
phase-separation porous structure obtained by dry-wet spinning
utilizing a principle of nonsolvent induced phase separation or
thermally induced phase separation, because a large number of fine
voids are present and the porosity is high.
[0292] In the case of the porous membrane having a fibrous texture
described in Patent Document 5, it is considered that stress during
stretching is dispersed by the fibrous texture oriented in the
longitudinal direction and stretching can be performed even though
the stretch ratio is as low as less than 2.0 times. However, it is
still very difficult to uniformly conduct high-ratio stretching of
2.0 times of more, and intensive studies on the cause thereof have
revealed that a fibrous texture has many narrowed portions and
because stress is concentrated at the narrowed portion during
stretching and the narrowed portion is therefore preferentially
stretched, the entire fibrous texture cannot be uniformly
stretched, making it impossible to increase the stretch ratio.
[0293] In contrast, the present inventors have found that it is not
a fibrous texture having a large number of narrowed portions
described in Patent Document 5, not a network structure described
in Patent Document 3, and not a spherical structure described in
Patent Document 4 but as long as it is a hollow fiber having a
columnar texture with uniform thickness, the entire columnar
texture can be uniformly stretched, and consequently, high-ratio
stretching of 2.0 times or more is made possible. Such uniform and
high-ratio stretching has yielded success in stretching and
orienting the molecular chain of the fluororesin-based polymer in
the longitudinal direction of the porous hollow-fiber membrane and
success in increasing the strength while maintaining high
pure-water permeation performance.
(c) Stretching
[0294] Finally, in the present invention, the porous hollow-fiber
membrane including a fluororesin-based polymer and having a
columnar texture, obtained by the method above, is stretched at a
high ratio, and the molecular chain of the polymer is thereby
oriented in the longitudinal direction of the hollow-fiber
membrane.
[0295] The stretch ratio is from 2.0 to 5.0 times, preferably from
2.5 to 4.0 times, more preferably from 2.5 to 3.5 times. If the
stretch ratio is less than 2.0 times, the molecular chain cannot be
sufficiently oriented by stretching, and if the stretch ratio
exceeds 5.0 times, reduction in the elongation increases.
[0296] The stretching temperature is preferably from 60 to
140.degree. C., more preferably from 70 to 120.degree. C., still
more preferably from 80 to 100.degree. C. If stretching is
performed in a low temperature atmosphere of less than 60.degree.
C., it is difficult to stably and homogeneously stretch the hollow
fiber. If the hollow fiber is stretched at a temperature exceeding
140.degree. C., since the temperature is close to the melting point
of the fluororesin-based polymer, the structure texture may be
melted to reduce the pure-water permeation performance.
[0297] Stretching in a liquid is preferred because of ease of
temperature control, but the stretching may also be performed in a
gas such as steam. As the liquid, water is simple and preferred,
but in the case of stretching the hollow fiber at about 90.degree.
C. or more, use of a low-molecular-weight polyethylene glycol, etc.
may also be preferably employed.
<Cross-Flow Filtration Method, Transmembrane Pressure
Difference>
[0298] In the cross-flow filtration, a raw liquid flows in through
the raw liquid inflow port 8 of the hollow-fiber membrane module
100, and the raw liquid is discharged from the raw liquid outlet
10. In addition, the filtered liquid is delivered to the upper part
of the hollow-fiber membrane module 100 through the hollow part of
the hollow-fiber membrane and discharged from the filtered liquid
outlet 9.
[0299] As described above, in the cross-flow filtration, the raw
liquid flows in parallel to the membrane surface. At this time, the
membrane surface linear velocity may be appropriately set in
accordance with the property of the raw liquid but is preferably
from 0.3 to 5 m/s.
[0300] The filtration flux at the time of cross-flow filtration may
be appropriately set in accordance with the property of the raw
liquid but is preferably from 0.1 to 5.0 m.sup.3/m.sup.2/d, more
preferably from 0.3 to 3.0 m.sup.3/m.sup.2/d.
[0301] In the filtration by a separation membrane, the clogging
state of the separation membrane can be determined from the
transmembrane pressure difference obtained by subtracting the
pressure on the filtered liquid side from the pressure on the raw
liquid side of the separation membrane, and in the case of dead-end
filtration, the transmembrane pressure difference can be calculated
using a manometer on the upstream side of the raw liquid inflow
port 8 of the hollow-fiber membrane module 100 and a pressure on
the downstream side of the filtered liquid outlet 9. If the
filtration flux is the same, when clogging of the separation
membrane develops, the transmembrane pressure difference rises.
However, in the case of cross-flow filtration, the pressure loss at
the time of passing of raw liquid through the raw liquid-side
passage of the hollow-fiber membrane module 100 is large, and the
transmembrane pressure difference calculated by the above-described
method also contains a pressure loss in the raw liquid-side passage
and therefore, is difficult to appropriately calculate.
Accordingly, denoting P1 as the filtered liquid-side pressure when
filtration is stopped while feeding raw liquid to the raw liquid
side of the hollow-fiber membrane module 100 and denoting P2 as the
filtered liquid-side pressure when filtration is preformed while
feeding raw liquid to the raw liquid side of the hollow-fiber
membrane module 100, .DELTA.P that is a value obtained by
subtracting P2 from P1 may be used as the transmembrane pressure
difference at the time of cross-flow filtration.
<Backwashing Method>
[0302] In the cross-flow filtration, backwashing can also be
conducted by periodically stopping filtration. Backwashing is
conducted to clean the membrane by feeding a backwashing solution
from the filtered liquid outlet 9 of the hollow-fiber membrane
module 100 and flowing the backwashing solution toward the outer
side from the inner side of the hollow-fiber membrane. When the
water permeability is recovered by backwashing, the filtration time
can be extended, and the frequency of chemical cleaning decreases,
so that the operation cost can be reduced. The backwashing can be
conducted with the filtered liquid, or other liquid such as water
may also be used.
[0303] The backwashing flux during backwashing may be appropriately
set in accordance with the property of raw liquid or the clogging
state of separation membrane but is preferably from 1.0 to 10.0
m.sup.3/m.sup.2/d, more preferably from 1.5 to 5.0
m.sup.3/m.sup.2/d. If the backwashing flux is less than 1.0
m.sup.3/m.sup.2/d, this is disadvantageous in that the cleaning
effect is decreased. If the backwashing flux exceeds 10.0
m.sup.3/m.sup.2/d, this is disadvantageous in that the power cost
increases and a large amount of solution needs to be used for
backwashing.
EXAMPLES
[0304] The present invention is described below by referring to
specific Examples, but the present invention is not limited by
these Examples in any way. Incidentally, the physical property
values related to the present invention can be measured by the
following methods.
(i) Pure-Water Permeation Performance
[0305] A compact module including 4 porous hollow-fiber membranes
and having an effective length of 200 mm was manufactured.
Distilled water was delivered to the module over 1 hour under the
conditions of a temperature of 25.degree. C. and a filtration
pressure difference of 16 kPa. and the amount (m.sup.3) of the
permeate obtained was measured, converted into a numerical value
per unit time (h) and unit membrane area (m.sup.3), further
converted in terms of a pressure (50 kPa), and used as the
pure-water permeation performance (m.sup.3/m.sup.2/h). The unit
membrane area was calculated from the average outside diameter and
the effective length of the hollow-fiber membrane.
(ii) Breaking Strength, Elongation at Break, Young's Modulus
[0306] Using a tensile tester (TENSILON (registered
trademark)/RTM-100, manufactured by Toyo Baldwin Co., Ltd.), a
sample having a measurement length of 50 mm was tested five or more
times by changing the sample in an atmosphere of 25.degree. C. at a
tensile speed of 50 mm/min, and the breaking strength, elongation
at break and Young's modulus were calculated by determining
respective average values.
(iii) Raman Orientation Parameter .nu.
[0307] The orientation parameter of the polyvinylidene fluoride
homopolymer in the hollow-fiber membrane was determine by the
following operation.
[0308] A cross-section in the longitudinal direction of the
hollow-fiber membrane was sliced by cutting with a microtome, and
10 columnar textures were selected per one hollow-fiber membrane.
For each columnar texture, the scattering intensity was measured by
Raman spectroscopy at 1 .mu.m intervals along the longitudinal
direction of columnar texture while checking the columnar texture
by an optical microscope.
[0309] The Raman orientation parameter of each texture was
calculated according to formula (1), and an average value of
respective Raman orientation parameters was defined as the Raman
orientation parameter .nu.. In addition, among 10 columnar textures
different from each other, a largest Raman orientation parameter
and a smallest Raman orientation parameter were selected,
respective average values were determined and denoted as maximum
Raman orientation parameter M and minimum Raman orientation
parameter m, and M/m was calculated.
Raman orientation parameter=(I1270/I840) parallel/(I1270/I840)
perpendicular (1)
wherein:
[0310] parallel condition: the longitudinal direction of the
hollow-fiber membrane is parallel to the polarization
direction,
[0311] perpendicular condition: the longitudinal direction of the
hollow-fiber membrane is orthogonal to the polarization
direction,
[0312] I1270 parallel: the intensity of Raman band at 1,270
cm.sup.-1 under parallel condition,
[0313] I1270 perpendicular: the intensity of Raman band at 1,270
cm.sup.-1 under perpendicular condition,
[0314] I840 parallel: the intensity of Raman band at 840 cm.sup.-1
under parallel condition, and
[0315] I840 perpendicular: the intensity of Raman band at 840
cm.sup.-1 under perpendicular condition.
[0316] The laser Raman spectrometer and measurement conditions are
as follows.
[0317] Apparatus: Jobin Yvon/Atago Bussan, T-64000
[0318] Conditions: [0319] Measurement mode: micro-Raman [0320]
Object lens: .times.100 [0321] Beam diameter: 1 .mu.m [0322] Light
source: Ar+laser/514.5 nm [0323] Laser power: 100 mW [0324]
Diffraction grating: Single 600 gr/mm [0325] Slit: 100 .mu.m [0326]
Detector: CCD/Jobin Yvon 1024.times.256
(iv) Thickness Uniformity
[0327] First, the hollow-fiber membrane was resin-embedded in an
epoxy resin and subjected to osmium staining treatment, and the
void portion was thereby filled with the epoxy resin. Next, using a
scanning electron microscope (SEM) equipped with a focused ion beam
(FIB), a face parallel to the short-side direction of the
hollow-fiber membrane was cut out using FIB, and FIB cutting and
SEM observation were repeatedly conducted 200 times at 50 nm
intervals toward the longitudinal direction of the hollow-fiber
membrane to obtain information at a depth of 10 .mu.m.
[0328] The thickness uniformity was determined by comparing a first
cross-section and a second cross-section each running in parallel
to the short-side direction of the hollow-fiber membrane, which
were obtained in the above-described continuous cross-sectional
observation using FIB. Here, 20 pairs of first cross-section and
second cross-section were selected such that these cross-sections
were faces parallel to each other and spaced 5 .mu.m apart.
[0329] First, in each cross-section, a portion including resin and
a void portion (epoxy portion) were distinguished, and the area of
the resin portion and the area of the void portion were measured.
Subsequently, the area of a portion where when the first
cross-section is projected onto the second cross-section from a
direction perpendicular to both cross-sections, the portion
including resin in the first cross-section and the portion
including resin in the second cross-section are overlapped (overlap
area), was determined.
[0330] The thickness uniformities in each pair were calculated as
values obtained by averaging thickness uniformities A and B
determined according to the following formulae (6) and (7). Since
20 average values of A and B are obtained, an average value
obtained from these 20 values was defined as the thickness
uniformity of the membrane.
[0331] In addition, the membrane was determined to have a columnar
texture when 16 pairs or more have a thickness uniformity of 0.50
or more, and determined to have a fibrous texture when 15 pairs or
less have the thickness uniformity above.
Thickness uniformity A=(overlap area)/(area of resin portion of
second cross-section) (6)
Thickness uniformity B=(overlap area)/(area of resin portion of
first cross-section) (7)
(v) Orientation Degree .pi. of Molecular Chain in Longitudinal
Direction of Hollow-Fiber Membrane
[0332] A hollow-fiber membrane was fixed to a fiber sample stage by
arranging its longitudinal direction to run vertically and
subjected to X-ray diffraction measurement (2.theta./.theta.
scanning, .beta. scanning) by using an X-ray diffractometer
(SmartLab for polymer, CuK.alpha. ray, manufactured by Rigaku
Corporation). First, it was confirmed by 2.theta./.theta. scanning
that a peak top is present at 2.theta.=20.4.degree.. Next, the
intensity in the range from 0.degree. up to 360.degree. in the
azimuth angle direction, relative to the diffraction peak at
2.theta.=20.4.degree., was measured by 0 scanning to obtain an
intensity distribution in the azimuth angle direction. Here, when
the ratio between the intensity at an azimuth angle of 180.degree.
and the intensity at an azimuth angle of 90.degree. was 0.80 or
less or was 1.25 or more, it is regarded that a peak is present and
by determining the width at a position of half the peak height
(half-width H) from the intensity distribution in the azimuth angle
direction, the orientation degree .pi. was calculated according to
the following formula (2). Since a minimum value of the intensity
in .beta. scanning was observed at 0.degree. and around
180.degree., a straight line passing these points was used as a
baseline.
Orientation degree .pi.=(180.degree.-H)/1800 (2)
(vi) Longitudinal Length and Short-Side Length of Columnar
Texture
[0333] With respect to the hollow-fiber membrane manufactured in
each working example, a cross-section along its longitudinal
direction was photographed at a magnification of 3,000 times by
means of a scanning electron microscope. On the image photographed,
10 columnar textures were randomly selected, and the longitudinal
length and short-side length of each texture were measured. Here,
as the longitudinal length of each columnar texture, the maximum
length in the longitudinal direction was measured. Furthermore, as
described above, a value obtained by dividing the longitudinal
length of each columnar texture by 1 .mu.m and rounding the
quotient down to the nearest integer is used as the number of
measurement points, and the short-side length of each columnar
texture was determined by measuring the length in the short-side
direction and calculating an average value thereof.
[0334] The photographing above was performed at 5 sites and by
determining the longitudinal length and short-side length for
arbitrary 10 columnar textures at each site, a total of 50
longitudinal lengths and a total of 50 short-side lengths were
obtained. Subsequently, an average value of a total of 50
longitudinal lengths was calculated and used as a representative
value of the longitudinal length, and an average value of a total
of 50 short-side lengths was calculated and used as a
representative value of the short-side length.
(vii) Porosity
[0335] With respect to arbitrary 20 cross-sections selected from 20
pairs of first cross-section and second cross-section obtained in
"(vi) Thickness Uniformity", i.e., a total of 40 cross-sections,
the porosity was determined according to the following formula (8)
by using the area of the resin portion and the area of the void
portion, and an average value thereof was used.
Porosity (%)={100.times.(area of void portion)}/{(area of resin
portion)+(area of void portion)} (8)
(viii) Occupancy of Texture
[0336] The occupancy of the texture was determined according to the
following formula (9) after taking a photograph of a cross-section
in the longitudinal direction of the hollow-fiber membrane by means
of a scanning electron microscope at a magnification of 3,000 times
in arbitrary 20 places, and an average value thereof was employed.
Here, the area of the entire photograph and the area occupied by a
texture were determined by printing the taken photograph on paper
and converting respective areas into the weight of paper
corresponding to the entire photograph and the weight of paper
corresponding to a texture portion cut out therefrom.
Occupancy (%)=((area occupied by each texture)/(area of entire
photograph)).times.100 (9)
(ix) Crystallization Temperature Tc of Fluororesin-Based Polymer
Solution
[0337] Using DSC-6200 manufactured by Seiko Instruments &
Electronics Ltd., a mixture having the same composition as the
composition of the membrane-forming polymer solution containing a
fluororesin-based polymer, a solvent, etc. was sealed in a sealing
type DSC container and uniformly dissolved by raising the
temperature to a dissolution temperature at a temperature rise rate
of 10.degree. C./min and holding the temperature for 30 minutes,
and a rise temperature of a crystallization peak observed in the
process of thereafter lowering the temperature at a temperature
drop rate of 10.degree. C./min was used as the crystallization
temperature Tc.
Reference Example 1
[0338] 35 wt % of a vinylidene fluoride homopolymer (KF1300,
produced by Kureha Corporation, weight average molecular weight:
417,000, number average molecular weight: 221,000) having a weight
average molecular weight of 417,000 and 65 wt % of y-butyrolactone
were dissolved at 150.degree. C. Tc of the thus-obtained vinylidene
fluoride homopolymer solution (i.e., raw material solution) was
46.degree. C.
[0339] For the pressurization and discharge of the raw material
solution, an apparatus having a double tube-type spinneret, a
piping connected to the spinneret, and two gear pumps disposed on
the piping was used. Within the piping between gear pumps, the raw
material solution was retained at 99 to 101.degree. C. for 15
seconds under a pressure of 2.5 MPa. Thereafter, while discharging
an aqueous 85 wt % .gamma.-butyrolactone solution through the inner
tube of the double tube-type spinneret, the raw material solution
was discharged through the outer tube. The raw material solution
was allowed to stay in a cooling bath at a temperature of
20.degree. C. containing an aqueous 85 wt % y-butyrolactone
solution for 20 seconds and thereby solidified.
[0340] The obtained hollow-fiber membrane had a columnar texture
with a thickness uniformity of 0.55, where the occupancy of
columnar texture was 85% and the occupancy of spherical texture was
15%.
[0341] The hollow-fiber membrane obtained above was then stretched
to 2.0 times at a stretching speed of 9%/sec in water at 95.degree.
C.
[0342] The hollow-fiber membrane after stretching was observed, as
a result, a columnar texture was recognized. Furthermore, the
hollow-fiber membrane had a columnar texture with a representative
value of longitudinal length of 16 .mu.m, a representative value of
short-side length of 2.1 .mu.m, and a thickness uniformity of 0.51,
where the porosity was 56%, the orientation degree .pi. of the
molecular chain of vinylidene fluoride homopolymer in the
longitudinal direction of the hollow-fiber membrane could not be
calculated, that is, the molecular chain was non-oriented, the
Raman orientation parameter .nu. was 1.82, the maximum Raman
orientation parameter M was 2.31, the minimum Raman orientation
parameter m was 1.32, and M/m was 1.8. Here, the outside diameter
of the hollow-fiber membrane obtained was 850 .mu.m, and the inside
diameter was 550 .mu.m. Furthermore, the breaking strength of the
hollow-fiber membrane was 26 MPa, and the pure-water permeation
performance was 1.0 m.sup.3/m.sup.2/hr.
Reference Example 2
[0343] A raw material solution was prepared in the same manner as
in Reference Example 1 except that the concentration of the
vinylidene fluoride homopolymer was changed to 39 wt %. Tc of the
raw material solution was 49.degree. C.
[0344] The raw material solution was retained at 99 to 101.degree.
C. for 20 seconds under a pressure of 2.5 MPa applied by means of
the same apparatus as in Reference Example 1 and then discharged
from the same double tube-type spinneret as in Reference Example 1.
The raw material solution discharged was allowed to stay in a first
cooling bath at a temperature of 5.degree. C. containing an aqueous
85 wt % .gamma.-butyrolactone solution for 10 seconds, further
allowed to stay in a second cooling bath at a temperature of
30.degree. C. containing an aqueous 85 wt % .gamma.-butyrolactone
solution for 40 seconds, and thereby solidified.
[0345] The obtained hollow-fiber membrane had a columnar texture
with a thickness uniformity of 0.69, where the occupancy of
columnar texture was 91% and the occupancy of spherical texture was
9%.
[0346] The hollow-fiber membrane obtained above was then stretched
to 2.4 times at a stretching speed of 142%/sec in water at
95.degree. C.
[0347] The hollow-fiber membrane after stretching had a columnar
texture with a representative value of longitudinal length of 22
.mu.m, a representative value of short-side length of 1.8 .mu.m,
and a thickness uniformity of 0.62, where the porosity was 54%, the
orientation degree .pi. of the molecular chain of vinylidene
fluoride homopolymer in the longitudinal direction of the
hollow-fiber membrane was 0.31, the Raman orientation parameter
.nu. was 2.53, the maximum Raman orientation parameter M was 3.08,
the minimum Raman orientation parameter m was 1.14, and M/m was
2.7. Here, the outside diameter of the hollow-fiber membrane
obtained was 850 .mu.m, and the inside diameter was 550 .mu.m.
Furthermore, the breaking strength of the hollow-fiber membrane was
35 MPa, and the pure-water permeation performance was 1.6
m.sup.3/m.sup.2/hr.
Reference Example 3
[0348] 42 wt % of a vinylidene fluoride homopolymer (KF1300,
produced by Kureha Corporation, weight average molecular weight:
417,000, number average molecular weight: 221,000) having a weight
average molecular weight of 417,000 and 58 wt % of
dimethylsulfoxide were dissolved at 130.degree. C. Tc of the
thus-obtained vinylidene fluoride homopolymer solution (i.e., raw
material solution) was 35.degree. C.
[0349] The raw material solution was retained at 78 to 80.degree.
C. for 20 seconds under a pressure of 2.5 MPa applied by means of
the same apparatus with gear pumps as in Reference Example 1.
Thereafter, while discharging an aqueous 90 wt % dimethylsulfoxide
solution through the inner tube of the double tube-type spinneret,
the raw material solution was discharged through the outer tube.
The raw material solution discharged was allowed to stay in a first
cooling bath at a temperature of -3.degree. C. containing an
aqueous 85 wt % dimethylsulfoxide solution for 10 seconds, further
allowed to stay in a second cooling bath at a temperature of
20.degree. C. containing an aqueous 85 wt % dimethylsulfoxide
solution for 50 seconds, and thereby solidified. The obtained
hollow-fiber membrane had a columnar texture with a thickness
uniformity of 0.72, where the occupancy of columnar texture was 95%
and the occupancy of spherical texture was 5%.
[0350] The hollow-fiber membrane obtained above was then stretched
to 2.4 times at a stretching speed of 125%/sec in water at
95.degree. C. The hollow-fiber membrane after stretching had a
columnar texture with a representative value of longitudinal length
of 22 .mu.m, a representative value of short-side length of 1.8
.mu.m, and a thickness uniformity of 0.70, where the porosity was
56%, the orientation degree .pi. of the molecular chain of
vinylidene fluoride homopolymer in the longitudinal direction of
the hollow-fiber membrane was 0.34, the Raman orientation parameter
.nu. was 2.96, the maximum Raman orientation parameter M was 3.31,
the minimum Raman orientation parameter m was 1.42, and M/m was
2.3. Here, the outside diameter of the hollow-fiber membrane
obtained was 850 .mu.m, and the inside diameter was 550 .mu.m.
Furthermore, the breaking strength of the hollow-fiber membrane was
29 MPa. and the pure-water permeation performance was 2.2
m.sup.3/m.sup.2/hr.
Reference Example 4
[0351] A raw material solution was prepared in the same manner as
in Reference Example 1 except that the concentration of the
vinylidene fluoride homopolymer was changed to 39 wt %. Tc of the
raw material solution was 49.degree. C.
[0352] The raw material solution was retained at 99 to 101.degree.
C. for 20 seconds under a pressure of 2.5 MPa applied by means of
the same apparatus as in Reference Example 1. Thereafter, the raw
material solution was discharged from the double tube-type
spinneret in the same manner as in Reference Example 1. The raw
material solution discharged was allowed to stay in a first cooling
bath at a temperature of 5.degree. C. containing an aqueous 85 wt
%/o .gamma.-butyrolactone solution for 10 seconds, further allowed
to stay in a second cooling bath at a temperature of 35.degree. C.
containing an aqueous 85 wt % .gamma.-butyrolactone solution for 50
seconds, and thereby solidified.
[0353] The obtained hollow-fiber membrane had a columnar texture
with a thickness uniformity of 0.68, where the occupancy of
columnar texture was 92% and the occupancy of spherical texture was
8%.
[0354] The hollow-fiber membrane obtained above was then stretched
to 1.8 times at a stretching speed of 2%/sec in water at 95.degree.
C.
[0355] The hollow-fiber membrane after stretching had a columnar
texture with a representative value of longitudinal length of 13
.mu.m, a representative value of short-side length of 1.9 .mu.m,
and a thickness uniformity of 0.66, where the porosity was 53%, the
orientation degree .pi. of the molecular chain of vinylidene
fluoride homopolymer in the longitudinal direction of the
hollow-fiber membrane could not be calculated, that is, the
molecular chain was non-oriented, the Raman orientation parameter
.nu. was 2.13, the maximum Raman orientation parameter M was 2.69,
the minimum Raman orientation parameter m was 1.65, and M/m was
1.6. Here, the outside diameter of the hollow-fiber membrane
obtained was 850 .mu.m, and the inside diameter was 550 .mu.m.
Furthermore, the breaking strength of the hollow-fiber membrane was
27 MPa, and the pure-water permeation performance was 0.7
m.sup.3/m.sup.2/hr.
Reference Example 5
[0356] A raw material solution was prepared in the same manner as
in Reference Example 1 except that the concentration of the
vinylidene fluoride homopolymer was changed to 36 wt %. Tc of the
raw material solution was 48.degree. C.
[0357] The raw material solution was pressurized in the same manner
as in Reference Example 1 and then discharged from the double
tube-type spinneret. The raw material solution discharged was
allowed to stay in a first cooling bath at a temperature of
10.degree. C. containing an aqueous 85 wt % .gamma.-butyrolactone
solution for 10 seconds, further allowed to stay in a second
cooling bath at a temperature of 20.degree. C. containing an
aqueous 85 wt % .gamma.-butyrolactone solution for 20 seconds, and
thereby solidified.
[0358] The obtained hollow-fiber membrane had a columnar texture
with a thickness uniformity of 0.64, where the occupancy of
columnar texture was 87% and the occupancy of spherical texture was
13%.
[0359] The hollow-fiber membrane obtained above was then stretched
to 2.4 times at a stretching speed of 44%/sec in water at
95.degree. C. The hollow-fiber membrane after stretching had a
columnar texture with a representative value of longitudinal length
of 18 .mu.m, a representative value of short-side length of 1.9
.mu.m, and a thickness uniformity of 0.60, where the porosity was
55%, the orientation degree .pi. of the molecular chain of
vinylidene fluoride homopolymer in the longitudinal direction of
the hollow-fiber membrane was 0.25, the Raman orientation parameter
.nu. was 2.35, the maximum Raman orientation parameter M was 2.84,
the minimum Raman orientation parameter m was 1.21, and M/m was
2.4. Here, the outside diameter of the hollow-fiber membrane
obtained was 850 .mu.m, and the inside diameter was 550 .mu.m.
Furthermore, the breaking strength of the hollow-fiber membrane was
26 MPa. and the pure-water permeation performance was 2.0
m.sup.3/m.sup.2/hr.
Reference Example 6
[0360] A raw material solution was prepared in the same manner as
in Reference Example 1. The raw material solution was retained at
99 to 101.degree. C. for 20 seconds under a pressure of 2.5 MPa
applied by means of the same apparatus as in Reference Example 1.
Thereafter, the raw material solution was discharged from the
spinneret in the same manner as in Reference Example 1. The raw
material solution discharged was allowed to stay in a cooling bath
at a temperature of 5.degree. C. containing an aqueous 85 wt %
.gamma.-butyrolactone solution for 20 seconds and thereby
solidified.
[0361] The obtained hollow-fiber membrane had a columnar texture
with a thickness uniformity of 0.42, where the occupancy of
columnar texture was 90% and the occupancy of spherical texture was
10%.
[0362] The hollow-fiber membrane obtained above was then stretched
to 1.5 times at a stretching speed of 44%/sec in water at
95.degree. C.
[0363] The hollow-fiber membrane after stretching had a columnar
texture with a representative value of longitudinal length of 12
.mu.m, a representative value of short-side length of 2.2 .mu.m,
and a thickness uniformity of 0.39, where the porosity was 56%, the
orientation degree .pi. of the molecular chain of vinylidene
fluoride homopolymer in the longitudinal direction of the
hollow-fiber membrane could not be calculated, that is, the
molecular chain was non-oriented, the Raman orientation parameter
.nu. was 1.01, the maximum Raman orientation parameter M was 1.03,
the minimum Raman orientation parameter m was 1.00, and M/m was
1.0. Here, the outside diameter of the hollow-fiber membrane
obtained was 850 .mu.m, and the inside diameter was 550 .mu.m.
Furthermore, the breaking strength of the hollow-fiber membrane was
11 MPa, and the pure-water permeation performance was 1.0
m.sup.3/m.sup.2/hr.
Reference Example 7
[0364] 36 wt % of a vinylidene fluoride homopolymer (KF1300,
produced by Kureha Corporation, weight average molecular weight:
417,000, number average molecular weight: 221,000) and 64 wt % of
.gamma.-butyrolactone were dissolved at 150.degree. C. Tc of the
vinylidene fluoride homopolymer solution was 48.degree. C. By
disposing two gear pumps, the solution was retained at 99 to
101.degree. C. for 20 seconds on a line therebetween under a
pressure of 2.0 MPa. Thereafter, the solution was discharged
through the outer tube of a double tube-type spinneret while
discharging an aqueous 85 wt % .gamma.-butyrolactone solution
through the inner tube of the double tube-type spinneret, then
allowed to stay in a cooling bath at a temperature of 25.degree. C.
containing an aqueous 85 wt % .gamma.-butyrolactone solution for 20
seconds, and thereby solidified. The obtained hollow-fiber membrane
had a columnar texture with a thickness uniformity of 0.62, where
the occupancy of columnar texture was 86% and the occupancy of
spherical texture was 14%.
[0365] The hollow-fiber membrane obtained above was then stretched
to 2.5 times in water at 95.degree. C. The hollow-fiber membrane
after stretching was observed, as a result, a columnar texture was
recognized. Furthermore, the hollow-fiber membrane had a columnar
texture with a representative value of longitudinal length of 16
.mu.m, a representative value of short-side length of 2.2 .mu.m,
and a thickness uniformity of 0.61, where the porosity was 55%, the
orientation degree .pi. of the molecular chain of vinylidene
fluoride homopolymer in the longitudinal direction of the
hollow-fiber membrane was 0.61, the Raman orientation parameter
.nu. was 3.12, and M/m was 3.1. In addition, the breaking strength
of the hollow-fiber membrane was 27 MPa, and the pure-water
permeation performance was 2.1 m.sup.3/m.sup.2/hr.
Reference Example 8
[0366] 38 wt % of a vinylidene fluoride homopolymer (KF1300,
produced by Kureha Corporation, weight average molecular weight:
417,000, number average molecular weight: 221.000) and 62 wt % of
.gamma.-butyrolactone were dissolved at 150.degree. C. Tc of the
vinylidene fluoride homopolymer solution was 51.degree. C. By
disposing two gear pumps, the solution was retained at 99 to
101.degree. C. for 20 seconds on a line therebetween under a
pressure of 2.0 MPa. Thereafter, the solution was discharged
through the outer tube of a double tube-type spinneret while
discharging an aqueous 85 wt % .gamma.-butyrolactone solution
through the inner tube of the double tube-type spinneret, then
allowed to stay in a first cooling bath at a temperature of
5.degree. C. containing an aqueous 85 wt % .gamma.-butyrolactone
solution for 10 seconds, further allowed to stay in a second
cooling bath at a temperature of 35.degree. C. containing an
aqueous 85 wt/o .gamma.-butyrolactone solution for 50 seconds, and
thereby solidified. The obtained hollow-fiber membrane had a
columnar texture with a thickness uniformity of 0.66, where the
occupancy of columnar texture was 91% and the occupancy of
spherical texture was 9%.
[0367] The hollow-fiber membrane obtained above was then stretched
to 3.5 times in water at 95.degree. C. The hollow-fiber membrane
after stretching had a columnar texture with a longitudinal length
of 28 .mu.m, a short-side length of 1.3 .mu.m, and a thickness
uniformity of 0.62, where the porosity was 61%, the orientation
degree .pi. of the molecular chain of vinylidene fluoride
homopolymer in the longitudinal direction of the hollow-fiber
membrane was 0.89, the Raman orientation parameter .nu. was 4.42,
and M/m was 5.1. In addition, the breaking strength of the
hollow-fiber membrane was 62 MPa, and the pure-water permeation
performance was 2.6 m.sup.3/m.sup.2/hr. FIG. 5 illustrates a
photograph of a cross-section in the longitudinal direction of the
hollow-fiber membrane, FIG. 7 illustrates the intensity
distribution in the azimuth angle direction at
2.theta.=20.4.degree. of the hollow-fiber membrane, and FIG. 8
illustrates the Raman orientation parameter at each measurement
site of the hollow-fiber membrane.
Reference Example 9
[0368] 40 wt % of a vinylidene fluoride homopolymer (KF1300,
produced by Kureha Corporation, weight average molecular weight:
417,000, number average molecular weight: 221,000) and 60 wt % of
dimethylsulfoxide were dissolved at 130.degree. C. Tc of the
vinylidene fluoride homopolymer solution was 30.degree. C. By
disposing two gear pumps, the solution was retained at 78 to
80.degree. C. for 20 seconds on a line therebetween under a
pressure of 2.0 MPa. Thereafter, the solution was discharged
through the outer tube of a double tube-type spinneret while
discharging an aqueous 90 wt % dimethylsulfoxide solution through
the inner tube of the double tube-type spinneret, then allowed to
stay in a first cooling bath at a temperature of -5.degree. C.
containing an aqueous 85 wt % dimethylsulfoxide solution for 10
seconds, further allowed to stay in a first cooling bath at a
temperature of 15.degree. C. containing an aqueous 85 wt %
dimethylsulfoxide solution for 30 seconds, and thereby solidified.
The obtained hollow-fiber membrane had a columnar texture with a
thickness uniformity of 0.72, where the occupancy of columnar
texture was 92% and the occupancy of spherical texture was 8%.
[0369] The hollow-fiber membrane obtained above was then stretched
to 3 times in water at 95.degree. C. The hollow-fiber membrane
after stretching had a columnar texture with a longitudinal length
of 27 .mu.m, a short-side length of 1.7 .mu.m, and a thickness
uniformity of 0.69, where the porosity was 64%, the orientation
degree .pi. of the molecular chain of vinylidene fluoride
homopolymer in the longitudinal direction of the hollow-fiber
membrane was 0.86, the Raman orientation parameter .nu. was 4.38,
and M/m was 5.1. In addition, the breaking strength of the
hollow-fiber membrane was 52 MPa, and the pure-water permeation
performance was 2.3 m.sup.3/m.sup.2/hr.
Reference Example 10
[0370] 40 wt % of a vinylidene fluoride homopolymer (KF1300,
produced by Kureha Corporation, weight average molecular weight:
417,000, number average molecular weight: 221,000) and 60 wt % of
dimethylsulfoxide were dissolved at 130.degree. C. Tc of the
vinylidene fluoride homopolymer solution was 30.degree. C. By
disposing two gear pumps, the solution was retained at 78 to
80.degree. C. for 20 seconds on a line therebetween under a
pressure of 2.0 MPa. Thereafter, the solution was discharged
through the outer tube of a double tube-type spinneret while
discharging an aqueous 90 wt % dimethylsulfoxide solution through
the inner tube of the double tube-type spinneret, then allowed to
stay in a first cooling bath at a temperature of -5.degree. C.
containing an aqueous 85 wt 6 dimethylsulfoxide solution for 10
seconds, further allowed to stay in a first cooling bath at a
temperature of 20.degree. C. containing an aqueous 85 wt %
dimethylsulfoxide solution for 50 seconds, and thereby solidified.
The obtained hollow-fiber membrane had a columnar texture with a
thickness uniformity of 0.72, where the occupancy of columnar
texture was 95% and the occupancy of spherical texture was 5%.
[0371] The hollow-fiber membrane obtained above was then stretched
to 4 times in water at 95.degree. C. The hollow-fiber membrane
after stretching had a columnar texture with a longitudinal length
of 40 .mu.m, a short-side length of 1.1 .mu.m, and a thickness
uniformity of 0.63, where the porosity was 66%, the orientation
degree .pi. of the molecular chain of vinylidene fluoride
homopolymer in the longitudinal direction of the hollow-fiber
membrane was 0.92, the Raman orientation parameter .nu. was 4.76,
and M/m was 6.2. In addition, the breaking strength of the
hollow-fiber membrane was 68 MPa, and the pure-water permeation
performance was 2.8 m.sup.3/m.sup.2/hr.
Reference Example 11
[0372] 38 wt % of a vinylidene fluoride homopolymer (KF1300,
produced by Kureha Corporation, weight average molecular weight:
417,000, number average molecular weight: 221.000) and 62 wt % of
.gamma.-butyrolactone were dissolved at 150.degree. C. Tc of the
vinylidene fluoride homopolymer solution was 51.degree. C. By
disposing two gear pumps, the solution was retained at 99 to
101.degree. C. for 20 seconds on a line therebetween under a
pressure of 2.0 MPa. Thereafter, the solution was discharged
through the outer tube of a double tube-type spinneret while
discharging an aqueous 85 wt % .gamma.-butyrolactone solution
through the inner tube of the double tube-type spinneret, then
allowed to stay in a cooling bath at a temperature of 5.degree. C.
containing an aqueous 85 wt % .gamma.-butyrolactone solution for 20
seconds, and thereby solidified. The obtained hollow-fiber membrane
had a fibrous texture with a thickness uniformity of 0.47, where
the occupancy of fibrous texture was 91% and the occupancy of
spherical texture was 9%.
[0373] The hollow-fiber membrane obtained above was then stretched
to 1.5 times in water at 95.degree. C. The hollow-fiber membrane
after stretching had a fibrous texture with a longitudinal length
of 15 .mu.m, a short-side length of 2.2 .mu.m, and a thickness
uniformity of 0.45, where the porosity was 63%, the molecular chain
of vinylidene fluoride homopolymer was non-oriented, the Raman
orientation parameter .nu. was 1.01, and M/m was 1.0. In addition,
the breaking strength of the hollow-fiber membrane was 14 MPa, and
the pure-water permeation performance was 2.3 m.sup.3/m.sup.3/hr.
FIG. 6 illustrates a photograph of a cross-section in the
longitudinal direction of the hollow-fiber membrane, and FIG. 7
illustrates the intensity distribution in the azimuth angle
direction at 2.theta.=20.4.degree. of the hollow-fiber
membrane.
Example 1
(Manufacture of Module)
[0374] The hollow fiber membrane of Reference Example 1 was
immersed in an aqueous 30 mass % glycerin solution for 1 hour and
then air-dried. A bundle of the hollow fiber membranes was sealed
with a silicone adhesive (produced by Dow Corning Toray Co., Ltd.,
SH850A/B, a mixture of two components mixed to afford a mass ratio
of 50:50) at one end.
[0375] On the surfaces of a polysulfone-made cylindrical case 3
(inside diameter: 50 mm, length: 500 mm) and a flow regulating
cylinder 12, the region to which a potting agent is bonded was
preliminarily filed with sandpaper (#80) and degreased with
ethanol. Thereafter, as illustrated in FIG. 3, the hollow-fiber
membrane bundle was packed inside of the cylindrical case 3 and the
flow regulating cylinder 12. At this time, the filling ratio of the
hollow-fiber membrane was set to be 40%, the hollow-fiber membrane
bundle was disposed such that the end part on the sealed side faces
a first end part (right-side end part of FIG. 3) of the cylindrical
case 3, which is defining the module upper-part side, and a potting
cap 14 was further attached. A potting cap 15 having 36 holes in
the bottom thereof was attached to a second end part (left-side end
part of FIG. 3) defining the module lower-part side. Then, 36 pins
13 were inserted into the holes in the bottom of the potting cap 15
and secured. The positions of the pins 13 were arranged in the same
fashion as the through holes of FIG. 2, and a module having potting
caps attached to both ends in this way was installed in a
centrifugal molding machine.
[0376] A polymeric MDI (produced by Huntsman Japan Co., Ltd.,
Suprasec 5025), a polybutadiene-based polyol (produced by Cray
Valley. Krasol LBH 3000), and 2-ethyl-1,3-hexanediol were mixed to
afford a mass ratio of 57:100:26. The obtained mixture (i.e.,
polyurethane resin solution) was put in a potting agent charger
16.
[0377] Subsequently, the centrifugal molding machine was rotated,
and each of the potting caps at both ends was filled with the
potting agent to form a first potting part 4 and a second potting
part 5. The potting agent charger 16 is split in two directions,
and the polyurethane resin solution was charged into the module
upper-part side (first end part) and the module lower-part side
(second end part) due to centrifugal force. The temperature in the
centrifugal molding machine was set to be 35.degree. C., and the
centrifugation time was set to be 4 hours.
[0378] After the centrifugation, the potting caps and pins were
removed, and the potting agent was cured at room temperature for 24
hours. Thereafter, the potting agent portion (B-B plane depicted in
FIG. 3) on the outer side of the module upper-part side (first end
part side) of the polysulfone-made cylindrical case 3 was cut with
a chip saw-type rotary blade to open the end face of the
hollow-fiber membrane. An upper cap 6 and a lower cap 7 were then
fixed to both ends of the polysulfone-made cylindrical case to
obtain a hollow-fiber membrane module 100.
[0379] After that, ethanol was delivered to the hollow-fiber
membrane module 100 and filtered to fill the pores of the
hollow-fiber membrane with ethanol. Subsequently. RO water was
delivered and filtered to replace ethanol with RO water.
(Filtration Test)
[0380] A budding yeast (Saccharomyces cerevisiae strain CM3260) was
cultured at 30.degree. C. for 24 hours in a liquid culture medium
containing 20 g/L of glucose, 5 g/L of ammonium sulfate, 0.59 g/L
of potassium chloride, 0.1 g/L of sodium chloride, 0.1 g/L of
calcium chloride, 0.5 g/L of magnesium sulfate heptahydrate, 0.02
g/L of uracil, 0.06 g/L of leucine, 0.02 g/L of histidine, and 0.04
g/L of tryptophane.
[0381] The resulting yeast culture fluid was subjected to
cross-flow filtration by the hollow-fiber membrane module 100. In
the cross-flow filtration, the membrane surface linear velocity was
set to be 2.0 m's, and the filtration flux was set to be 1
m.sup.3/m.sup.2/d. Subsequently, backwashing with the filtered
liquid was performed. The backwashing flux was set to be 2
m.sup.3/m.sup.2/d. After that, air scrubbing was performed by
feeding a compressed air at 6 L/min through the module lower part.
The filtration time, backwashing time, and air scrubbing time per
cycle were set to be 28 minutes, 1 minute, and 1 minute,
respectively, and with a cycle consisting of cross-flow filtration,
backwashing and air scrubbing, 10 cycles were repeated. Denoting
.DELTA.P1 as the transmembrane pressure difference 1 minute after
starting cross-flow filtration of the first cycle and .DELTA.P2 as
the transmembrane pressure difference 27 minutes after starting
cross-flow filtration of the tenth cycle, the rising degree
.DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.5.
Example 2
[0382] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 1 except that the filling ratio of the
hollow-fiber membrane was changed to 60%, and cross-flow filtration
of the yeast culture fluid was performed, as a result, the rising
degree .DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.2.
Example 3
[0383] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 1 except that the filling ratio of the
hollow-fiber membrane was changed to 75%, and cross-flow filtration
of the yeast culture fluid was performed, as a result, the rising
degree .DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.0.
Example 4
[0384] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 2 except that the hollow-fiber membrane
of Reference Example 2 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, the rising degree
.DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.2.
Example 5
[0385] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 2 except that the hollow-fiber membrane
of Reference Example 3 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, the rising degree
.DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.1.
Example 6
[0386] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 2 except that the hollow-fiber membrane
of Reference Example 4 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, the rising degree
.DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.2.
Example 7
[0387] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 2 except that the hollow-fiber membrane
of Reference Example 5 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, the rising degree
.DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.1.
Comparative Example 1
[0388] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 1 except that the filling ratio of the
hollow-fiber membrane was changed to 25%, and cross-flow filtration
of the yeast culture fluid was performed, as a result, the rising
degree .DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
3.4, revealing early clogging of the hollow-fiber membrane.
Comparative Example 2
[0389] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 2 except that the hollow-fiber membrane
of Reference Example 6 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, breakage of the
hollow-fiber membrane and leakage of raw liquid into filtered
liquid occurred.
Example 8
(Manufacture of Module)
[0390] The hollow fiber membrane of Reference Example 7 was
immersed in an aqueous 30 mass % glycerin solution for 1 hour and
then air-dried. A bundle of the hollow fiber membranes was sealed
with a silicone adhesive (produced by Dow Corning Toray Co., Ltd.,
SH850A/B, a mixture of two components mixed to afford a mass ratio
of 50:50) at one end.
[0391] On the surfaces of a polysulfone-made cylindrical case 3
(inside diameter: 50 mm, length: 500 mm) and a flow regulating
cylinder 12, the region to which a potting agent is bonded was
preliminarily filed with sandpaper (#80) and degreased with
ethanol. Thereafter, as illustrated in FIG. 3, the hollow-fiber
membrane bundle was packed inside of the cylindrical case 3 and the
flow regulating cylinder 12. At this time, the filling ratio of the
hollow-fiber membrane was set to be 41%, the hollow-fiber membrane
bundle was disposed such that the end part on the sealed side faces
a first end part (right-side end part of FIG. 3) of the cylindrical
case 3, which is defining the module upper-part side, and a potting
cap 14 was further attached. A potting cap 15 having 36 holes in
the bottom thereof was attached to a second end part (left-side end
part of FIG. 3) defining the module lower-part side. Then, 36 pins
13 were inserted into the holes in the bottom of the potting cap 15
and secured. The positions of the pins 13 were arranged in the same
fashion as the through holes of FIG. 2, and a module having potting
caps attached to both ends in this way was installed in a
centrifugal molding machine.
[0392] A polymeric MDI (produced by Huntsman Japan Co., Ltd.,
Suprasec 5025), a polybutadiene-based polyol (produced by Cray
Valley. Krasol LBH 3000), and 2-ethyl-1,3-hexanediol were mixed to
afford a mass ratio of 57:100:26. The obtained mixture (i.e.,
polyurethane resin solution) was put in a potting agent charger
16.
[0393] Subsequently, the centrifugal molding machine was rotated,
and each of the potting caps at both ends was filled with the
potting agent to form a first potting part 4 and a second potting
part 5. The potting agent charger 16 is split in two directions,
and the polyurethane resin solution was charged into the module
upper-part side (first end part) and the module lower-part side
(second end part) due to centrifugal force. The temperature in the
centrifugal molding machine was set to be 35.degree. C., and the
centrifugation time was set to be 4 hours.
[0394] After the centrifugation, the potting caps and pins were
removed, and the potting agent was cured at room temperature for 24
hours. Thereafter, the potting agent portion (B-B plane depicted in
FIG. 3) on the outer side of the module upper-part side (first end
part side) of the polysulfone-made cylindrical case 3 was cut with
a chip saw-type rotary blade to open the end face of the
hollow-fiber membrane. An upper cap 6 and a lower cap 7 were then
fixed to both ends of the polysulfone-made cylindrical case to
obtain a hollow-fiber membrane module 100.
[0395] After that, ethanol was delivered to the hollow-fiber
membrane module 100 and filtered to fill the pores of the
hollow-fiber membrane with ethanol. Subsequently. RO water was
delivered and filtered to replace ethanol with RO water.
(Filtration Test)
[0396] A budding yeast (Saccharomvces cerevisiae strain CM3260) was
cultured at 30.degree. C. for 24 hours in a liquid culture medium
containing 20 g/L of glucose, 5 g/L of ammonium sulfate, 0.59 g/L
of potassium chloride, 0.1 g/L of sodium chloride, 0.1 g/L of
calcium chloride, 0.5 g/L of magnesium sulfate heptahydrate, 0.02
g/L of uracil, 0.06 g/L of leucine, 0.02 g/L of histidine, and 0.04
g/L of tryptophane.
[0397] The resulting yeast culture fluid was subjected to
cross-flow filtration by the hollow-fiber membrane module 100. In
the cross-flow filtration, the membrane surface linear velocity was
set to be 2.5 m/s, and the filtration flux was set to be 1
m.sup.3/m.sup.2/d. Subsequently, backwashing with the filtered
liquid was performed. The backwashing flux was set to be 2
m.sup.3/m.sup.2/d. The filtration time and backwashing time per
cycle were set to be 29 minutes and 1 minute, respectively, and
with a cycle consisting of cross-flow filtration and backwashing,
10 cycles were repeated. Denoting .DELTA.P1 as the transmembrane
pressure difference 1 minute after starting cross-flow filtration
of the first cycle and .DELTA.P2 as the transmembrane pressure
difference 28 minutes after starting cross-flow filtration of the
tenth cycle, the rising degree .DELTA.P2/.DELTA.P1 of transmembrane
pressure difference was 2.7.
Example 9
[0398] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 8 except that the filling ratio of the
hollow-fiber membrane was changed to 60%, and cross-flow filtration
of the yeast culture fluid was performed, as a result, the rising
degree .DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.4.
Example 10
[0399] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 8 except that the filling ratio of the
hollow-fiber membrane was changed to 75%, and cross-flow filtration
of the yeast culture fluid was performed, as a result, the rising
degree .DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.1.
Example 11
[0400] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 9 except that the hollow-fiber membrane
of Reference Example 8 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, the rising degree
.DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.4.
Example 12
[0401] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 9 except that the hollow-fiber membrane
of Reference Example 9 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, the rising degree
.DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.5.
Example 13
[0402] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 9 except that the hollow-fiber membrane
of Reference Example 10 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, the rising degree
.DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
2.4.
Comparative Example 3
[0403] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 8 except that the filling ratio of the
hollow-fiber membrane was changed to 25%, and cross-flow filtration
of the yeast culture fluid was performed, as a result, the rising
degree .DELTA.P2/.DELTA.P1 of transmembrane pressure difference was
3.9, revealing early clogging of the hollow-fiber membrane.
Comparative Example 4
[0404] A hollow-fiber membrane module 100 was manufactured by the
same method as in Example 9 except that the hollow-fiber membrane
of Reference Example 11 was used, and cross-flow filtration of the
yeast culture fluid was performed, as a result, breakage of the
hollow-fiber membrane and leakage of raw liquid into filtered
liquid occurred.
TABLE-US-00001 TABLE 1 Rising Degree Maximum Minimum of Breaking
Raman Raman Raman Trans- Strength Orien- Orien- Orien- Orien-
Thick- Pure-water membrane Filling of tation tation tation tation
ness Young's Permeation Pressure Ratio Membrane Parameter Parameter
Parameter Degree Unifor- Modulus Performance Difference Membrane
(%) (MPa) .nu. M m M/m .pi. mity (GPa) (m.sup.3/m.sup.2/h)
.DELTA.P2/.DELTA.P1 Breakage Example 1 40 26 1.82 2.31 1.32 1.8
non- 0.51 0.26 1.0 2.5 none oriented Example 2 60 26 1.82 2.31 1.32
1.8 non- 0.51 0.26 1.0 2.2 none oriented Example 3 75 26 1.82 2.31
1.32 1.8 non- 0.51 0.26 1.0 2.0 none oriented Example 4 60 35 2.53
3.08 1.14 2.7 0.31 0.62 0.24 1.6 2.2 none Example 5 60 29 2.96 3.31
1.42 2.3 0.34 0.70 0.35 2.2 2.1 none Example 6 60 27 2.13 2.69 1.65
1.6 non- 0.66 0.28 0.7 2.2 none oriented Example 7 60 26 2.35 2.84
1.21 2.4 0.25 0.60 0.22 2.0 2.1 none Comparative 25 26 1.82 2.31
1.32 1.8 non- 0.51 0.26 1.0 3.4 none Example 1 oriented Comparative
60 11 1.01 1.03 1.00 1.0 non- 0.39 0.16 1.0 -- breakage Example 2
oriented occurred
TABLE-US-00002 TABLE 2 Rising Breaking Degree of Strength Raman
Porosity of Pure-water Transmembrane Filling of Orientation
Orientation Hollow-Fiber Permeation Pressure Ratio Membrane Degree
Thickness Parameter Membrane Performance Difference Membrane (%)
(MPa) .pi. Uniformity .nu. (%) (m.sup.3/m.sup.2/h)
.DELTA.P2/.DELTA.P1 Breakage Example 8 41 27 0.61 0.61 3.12 55 2.1
2.7 none Example 9 60 27 0.61 0.61 3.12 55 2.1 2.4 none Example 10
75 27 0.61 0.61 3.12 55 2.1 2.1 none Example 11 60 62 0.89 0.62
4.42 61 2.6 2.4 none Example 12 60 52 0.86 0.69 4.38 64 2.3 2.5
none Example 13 60 68 0.92 0.63 4.76 66 2.8 2 4 none Comparative 25
27 0.61 0.61 3.12 55 2.1 3.9 none Example 3 Comparative 60 14
non-oriented 0.45 1.01 63 2.3 -- breakage Example 4 occurred
[0405] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope of the
invention. This application is based on Japanese Patent Application
(Patent Application No. 2016-108318) filed on May 31, 2016 and
Japanese Patent Application (Patent Application No. 2016-125527)
filed on Jun. 24, 2016, the contents of which are incorporated
herein by way of reference.
INDUSTRIAL APPLICABILITY
[0406] The hollow-fiber membrane module of the present invention
can be used for water purification treatment, industrial water
treatment, wastewater treatment, seawater desalination, and
treatments of various liquids such as fermentation liquid, food and
beverage.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
[0407] 100 Hollow-fiber membrane module [0408] 1 Hollow-fiber
membrane [0409] 2 Hollow-fiber membrane bundle [0410] 3 Cylindrical
case [0411] 4 First potting part [0412] 5 Second potting part
[0413] 6 Upper cap [0414] 7 Lower cap [0415] 8 Raw liquid inflow
port [0416] 9 Filtered liquid outlet [0417] 10 Raw liquid outlet
[0418] 11 Through hole [0419] 12 Flow regulating cylinder [0420] 13
Pin [0421] 14 Potting cap (first end part) [0422] 15 Potting cap
(second end part) [0423] 16 Potting agent charger [0424] 17
Columnar texture
* * * * *